So I have confirmed this with my advisor this afternoon, he graduated from a very top-ranked program (top 5). He mentioned that it is very common for top-ranked schools to accept more students each year just so they have enough of TAs to teach lab and recitation sections.
Then, the department will make it extraordinarily difficult for you to pass your PhD candidacy exams in your third year, a lot of students had to leave the program because of that, It's pretty crazy I know .