1. jackpirate mentioned more filler courses
2. The internet? If you had a bad professor and a poorly written textbook back in the day, you were kind of screwed outside of working with friends. Today the internet offers pieces of courses and HW examples from all over the world, and things like Coursera and MIT opencourse software are available.
3. MORE AP and IB classes allow students to take college level courses but alot of kids at my college for example, in stead of carrying over the credits, chose to retake Calc I & II, even though they had ACED the AP exams, giving them a freshman year filler of courses that were relatively a breeze for them. I don't believe as many students took AP and IB classes back then. It seems to be the norm nowadays to use this AP/IB edge to actually help yourself ease into college courses be essentially retaking them. Besides, graduating early from a 4 year university is not encouraged, most kids at that level also want the full 4 year experience so their motivation to transfer the credits goes down.
If you think about it, this gives the private universities grades a double edge. you are more likely to be accepted into a competitive private university if you are smart enough to be acing college level courses in highschool, and once you get to the private university, you breeze through those courses by essentially retaking them.
Most students used a combination of all three of these things and obtained 3.8-4.0 Freshman year, allowing for more slack when the harder courses started to come in.
4. Lets not forget aderall meds are handed out like candy these days to help kids work harder longer. Use of these drugs at top private universities is rampant. In many cases, parents seek out medication for their kids claiming ADHD when really what they are saying is "my kid is not working hard enough to meet up to my standards and because he must be as smart as I am that must be because he is distracted with video games, and therefore has ADHD". Many kids seek it out for themselves. Roughly 1/3rd of the kids I knew were taking aderall regularly (either with or without prescription) to pull all nighters on a regular basis to keep up in school.
one of many easily googled sources: http://www.nytimes.com/2012/06/10/education/seeking-academic...
5. As for the difference between public and private schools, competitiveness and cheating (illegal use of adderall included) is contagious and I definitely think competition, cheating, use of drugs to work harder longer (as opposed to enjoyment and recreation), seems to exponentiate itself amongst more competitiveness students, which statistically, are more likely to end up at more competitive schools.
6. Overall, more and more kids are going to college each year, but interestingly, the applicant pools for public universities does increase, the applicant pool for top private universities is exponential each year, allowing private schools to choose the cream of the crop, potentially creating more of a gap between the general caliber of students between private and public universities.
Anecdotal sure but to give you an idea, my friend who went to Johns Hopkins told me stories of kids ripping pages out of textbooks in the libraries so other students had no access, and I've heard this happens at top law schools as well. I witnessed in my private university even as a freshman. In many cases these are kids whose entire identities and their parents value of them depend on acing through ivy league schools, and failure (failure in these case being Bs)is not an option. I can't speak for the average public school, but I feel as if this mindset is less likely to be the norm here.
one of many sources to help support pt 5 & 6: http://nypost.com/2013/08/25/tutor-reveals-ivy-admissions-ma...
7. Someone else mentioned visa students who are TAs are pressured to give good grades for good reviews. With underclassmen on student visas, the competition seems to go up and they are in my experience more likely to engage in competitive behavior or cheating or aderall abuse because the stakes of failing out are also alot higher for them, but it seems to have the opposite effect if you are an undergrad versus be a grad grading TA trying to get good reviews from undergrads.
Anecdotal story, but proves that not all private schools use grade inflation liberally:
At my college, which is a private university ranked top 50, goes up and down each year by 20 or so in rankings, we are not top 5 but usually in top 25, grading is really tough and we were actually ranked top 3 toughest graders lumped in with MIT, except MIT exempts students from their entire freshman GPA (I've heard) to help with suicide and other such issues. Twice I was curved down because grades were so good we were suspected of cheating. The average GPA at my college was 2.7 when I graduated in 2012 and that is after 10% of students fail out. I wish I experienced more grade inflation at my school!
In the three cases of severe grade inflation during my undergrad career, the averages on the exams were 20/100 or 40/100, and they had to curve up 30-40 pts just get a third of the class to pass, and still that meant most kids got Bs or Cs. I'll never forget when we got 18/100 average for 64 kids in a class and we were all Engineering Majors. Different topic but I believe that results from hiring Professors who are better at research (brings in funding) than they are at teaching. Theres no reason statstically why the average of 64 students junior year of one of the hardest Engineering Majors at a top Engineering School should all be failing unless of course the Professor sucks at teaching.
So in this case, there was severe grade inflation, but that did not results in higher GPAs overall relative to other schools, it just helped A Professor not get in trouble for being responsible for causing the entire Electrical Engineering Department to fail and have to retake her course, which would have definitely put this Professor under review...