What does the “Current Period” data usage number mean? In the Settings app, under Cellular > Cellular Data Usage, I see a “Current Period” value of 6.8 GB. However, I’ve checked this number several times over the years, at all different points in my cellular billing cycle, and it always hovers around 6 GB. Is this really the amount of data that I’ve transferred using my data plan since my last bill? If not, what does the number mean?
Current Period refers to the period of time since the last reset. This is nothing to do with the cellular plan. You can see the beginning date of the current period by scrolling to the bottom and looking at the date under the reset option.

If you have never reset the counter, it has been counting up like the odometer of a car since you started the OS at a clean restore.