Schools don't become more selective. They will often pace themeselve's knowing that they have, let's say, 600 offers to make over the course of 5 months of interviews. They are going to make maybe 100 offers per month with the last 100 offers going to those who were on the bubble .
I have also likened this to a broad staircase. Your application is reviewed and you are assigned to a group on a specific stair. Starting at the top, those on the stairs are invited to interview. As more applicants arrive, they are assigned to stairs above or below you or on the same stair. Eventually, your stair may be invited to interview as all those above you have been cleared.
Inside the building, you are faced with another broad staircase and are assigned to a stair based on your stair outside. However, depending on your interview performance, you may be reassigned up a step or two, down a step or two, or relegate to the bottom of the staircase. When offers get made, they are made to the top x% of those who have interviewed.
I hope that this might help you picture what the process is like. If you are good enough to be admitted, you will eventually be at the top of the staircase, regardless of when you stepped foot on it.
