Why did i go over my data even though i had it off

I have an iPhone 5 with a 300mb data plan on AT&T, and i noticed i was almost going to go over it (i had 297 of 300mb) so i turned off my cellular usage off in the settings and just used my home Wi-Fi. But today i woke up and i got a message from AT&T saying that i had gone over my limit by 7mb! WHY? I had it off and never turned it on… Also, What happens if i go over my data?

You are charged extra when you go over your limit.

Was the 297 in real time or delayed? You could have gone over 300 before you even saw you were at 297.

The app is a little delayed before it shows you the results.

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