It's not actually an overlap or graphical error.

For better or for worse, the actual explanation is this:

In game, the value displayed is the value sent to the car as an input, while in the options screen the value displayed is a sort of raw value.

Here's the part that might be controversial: The value sent to the car only uses 90% of the pedal's range. That is, the value sent into the car goes from 0 to 1 as the controller axis goes from 5% to 95%.

This is the case for Throttle, Brake and Handbrake. The 5% margins may sound quite large but I think this was set a very long time ago to avoid any possibility of an issue from controllers not producing the full output range. I don't know about these days but that was common in the past.

For Clutch it's different, it goes from 0 to 1 as the controller goes from 5% to 65%. This is to make sure the biting point is a significant distance away from fully floored clutch pedal. So the clutch must be lifted 35% to start pulling. Again, set a very long time ago to make the clutch have a reasonably realistic characteristic, although in reality the biting point varies between vehicles.


Final statement:

It's not really a bug at all, but there may be a case for an improvement suggestion to be made.

For purposes of checking your potentiometers I think you should look only in the controls screen, not in game.