Your data usage won't always update immediately. It can take a while for reports from various towers/exchanges to get back to Verizon and be aggregated to your account. Sometimes, data also isn't reported until something ends the data session (like your phone going to sleep, being turned off, etc.), so that could also account for the difference. Finally, the total data count on your phone may also include data used on WiFi, which obviously won't show up on your Verizon bill.