That's curious, because for my first month of service, I was seeing just the opposite. My phones data usage was over 100mb higher than VZs reported usage. My billing cycle just ended yesterday, with VZ reporting 1.6Gb of data usage and my phone reporting 1.7Gb. Not sure what the 100mb difference is from, though it was off by that much within the first few days of using the phone, and over half of my total data usage came from the first few days of usage. Fairly typical for a new phone. My monthly usage tends to hover around 500-600mb a month, occasionally getting close to 1gb, but very rarely going over that.
I'll be keeping an eye on both numbers over the next few months to see if there's any trend.