Each country has a different idea of what matters for tax purposes. The IRS doesn’t necessarily have all the information that would be relevant to calculate taxes for most people (you have to include that information when you file, so they are in a position to check). They know your income and could probably figure out other wage-earners in the household based on their addresses. They don’t necessarily know all the people you provided more than 50% of their support for the year, or how much you paid on interest for mortgage payments (assuming you pay a mortgage), or how much you spent on deductible medical expenses, or deductible business expenses, or whether a child was born (presumably you’re paying 50+% of the baby’s expenses) and which month that happened, or whether somebody moved out and you no longer pay 50+% of their support, etc.

The IRS does a terrible job with the information it does have. One year, I had a typo in my daughter’s social security number. The IRS sent me a note that her number didn’t match the previous year’s return. I assume that they believed the previous year’s number was right, but instead of saying “we’ve amended the return by using the social security number that appeared on last year’s return,” they said “we’ve amended the return by removing that dependent and recalculating taxes; if you disagree with your higher bill, you’ll need to send us the following paperwork with the correct social security number” (of course, “correct” meant “the number we already have on file” more or less, but I had to figure out what that number was by looking at my own copy of the previous year’s return). Luckily, the previous year’s number was correct; otherwise I would have had to amend the previous return and then refile the current year’s.