This is something weird that I don't quite understand.

The trip computer in my TJ "resets" itself when it reaches 20 hours of driving. At 19.55 it beeps at me, then at 20.00 it cycles back to 0 and starts again.

Is there a reason for this?

as far as i know... thats just what happens.... it doesnt go past 20 hours

as far as i know... thats just what happens.... it doesnt go past 20 hours
Spot on.
Here's the all the maximums.

Read your manual it tells you that in there :)

Nothing wrong with it at all