For what it's worth, my usage reported by Verizon is typically within about 5% of what my phone reports. I just double checked and right now Verizon says I've used 2345MB and my phone says 2.26GB.
The difference between those two numbers is 0.09 GB. 0.09 is 3.8% of 2.35, or 4.0% of 2.26, so whichever way you look at it, it would be correct to say the difference is "within 5%".