There's a few things that could affect what your phone is showing.
Do you use your phone on Wifi? If you do the phones usage will reflect not only what you used with Verizon out of your data package but also what you use when your connected to wifi.
Verizon.com I believe has a certain amount of time to catch up. I am not 100% sure what their time is but to my knowledge its not "real time" data. If you have any other questions about your data I would call customer care they would have the specifics on the time and be able to help you out.
If you had used 16GB of data bandwidth on a limited plan then Verizon would have called you before then.
So: the answer seems to be it includes wifi data bandwidth consumption as well.
How long have you had your iPhone? If you have never reset your cellular usage in settings then it may be showing how much you have used since you began using the device...not the usage for just this billing period.