Bla1ze brought this to our attention and while we're likely not the first to notice this, we haven't found or figured a good answer for it either. If you have a low data usage on your iPhone (in the MBs rather than GBs) and you do a reboot, when your iPhone comes back up, the Cellular Network Data meter will be lower.

Above are two screen shots taken right before and right after a reboot, which show the difference.

We've heard before that the usage meter isn't accurate, can be gamed, and shouldn't be used in place of your carrier's official record (especially if you're concerned about data charges), but if anyone has any insight into what's going on (or not going on) with regards to the iPhone's inner metering, drop us a comment and let us know.