Your findings are completely consistent with what people have been saying many, many times over the past few months. If something doesn't get noticed quickly on "newest" then it can sink without trace, regardless of its inherent value. Unless you get some friends to check your submissions and upvote them if appropriate, it's very hit-n-miss.

It's mostly because of this that I have always visited "newest" which then also means that I recognise things when seeing them second time around. Recently I've stopped bothering marking repeats and duplications, in part because some people find it deeply irritating, in part because it seems unvalued, and, yes, in part because it drags down my average karma. If one of the metrics PG has put in place gives me a poor grade because of some activity, I'll think twice about continuing.

So certainly your hypothesis "a" seems true. Not sure about "b" because it depends on what you mean by "small fraction," but I would agree with that too.