IOS – Bug with iPhone data usage counter

You need to also turn off background updates, which is where the bulk of the data usage is likely occurring.

Settings> General > Background App Refresh > Background App Refresh Off. Of course, you can choose which apps you wish to turn off individually for background refresh instead.

Don't forget that if you are worried about cell data usage, you can also put your phone in Airplane mode, which turns off all radios, but then manually turn on Wifi. This will force your phone to use only Wifi, as the cell radios are off. Simply turn off Airplane mode when you wish to use the cell radio again.

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.