My iPhone 5 gives me unlimited data?

I have an iPhone 5 with an AT&T $25 pay as you go plan which includes 200 minutes and unlimited texting. But for some reason my 4G data works and I get unlimited Internet for no charge. My iPhone 4 did not do this.
My best friend has an iPhone 5c that used to have a sprint plan until her parents deactivated it. She kept the sprint sim card in it & for some odd reason she got free unlimited 4G data on her phone too. Her texting and calling doesn't work though, just the data. Does anyone know why this happens? None of my other friends have told me this happened to them

That website above

I'd check with AT&T about that. They got rid of unlimited data plans a while ago so its very likely you don't have that & you may get a bad bill later on thinking this was the case.