I know it seems weird to think that there's some sort of limit to unlimited data -- but let me explain (especially since as of 3 minutes ago, I was just as confused as you might be.)
If everyone had unlimited data, phones and other smart devices would be slow AF since everyone would be using a ton of data, on the same network, at the same time. It'd be miserable. So, what different cell phone providers do, is put a throttling limit on. So basically, once you hit a certain data usage limit, the provider will slow your internet down a bit to keep it fair for everyone else.
(Because, think about it, how annoyed would you be if you're getting slowed down when you use 1-2 GB a month, yet someone is using 120 GB a month and getting the highest speed. Annoying, right?)
Well, today, T-Mobile is raising their throttling limit from 32 GB (which is already the highest out of any provider), to 50 GB!!