Battery % Drops After TurnOff

When you turn the phone off it doesn’t just stop suddenly. iOS “tells" all of the open apps that the phone is shutting down, and gives them time to save any unsaved work; in addition the cellular subsystem “tells” the cellular network that the phone cannot be reached. Depending on what apps you have been using, this can take up to 30 seconds, and it uses energy. You may notice that you can’t turn it back on immediately, because it has to finish the shutdown process before you can restart it.

No, it all depends on how you use your phone, how you charge it and, quite frankly, the combination of chemicals in your battery and how they react over time.

No two phones will react quite the same.

But the biggest take away is - don't worry about it.

Aside from checking eleven or so months after purchase and checking whether it has fallen below 80%, there's no need to track what it is until it does fall to 80% or below, at which point you may want to consider having it replaced.

Tracking it with regularity doesn't buy you anything.

Plus when booting it has to do all the same in reverse - register with the cell tower and retrieve stored email and SMS messages.