That's where the 2 thermostats and dip tube come into play:

If the dip tube is working, it's putting the cold water into the bottom of the tank which helps stratify hot water at the top where it gets drawn off. And the lower thermostat will kick on to heat that cold makeup water to help maintain the tank temperature.

Once too much hot water is used and the lower element can't keep up, then it shuts off and the upper element kicks on to re-heat water near the draw-off as quickly as possible.