How do Data Plans work for iphone 4?

I'm connected to Wi-Fi and playing an app that doesn't connect to internet, but then I checked my data plan and it just rocketed form about 2.00 to 6.00 out of 300. I know that isn't close to the limit, but I just want to know why it did that.

Sometimes if your are not connected to Wi-Fi & you get notification like fb or instagram it will charge you even if you don't click, on the noticication