Data Usage Different from Phone Company

I dont know if its Verizon or if its the app but the data usage is different from each other. On the app its lower then Verizon and I even went as far as putting a higher number in the app and its still less then Verizons. So either Verizon is cheating us out of our data plan or the app is off.

FireGal183 about Data Monitor - Manage Data Usage in Real Time