iphone updated without completing its progress bar

Hello! So I recently updated my iphone, but the progress bar with the apple logo didn’t complete and from half way restarted the progress. Same thing happened the second time as the progress bar didn’t complete itself.

Yet this time the phone turned on and when I checked the updates in settings it showed me that the phone has been updated.

I am just wondering that if the progress bar was not completed how could the phone be updated?