It's the polarity, not the TPS, nor ECU

You are correct in your understanding of the operation, which IF you have an L8 (most likely) means someone has likely used a 2WD loom (so the outer wires on the TPS need inverting) or just fitted it wrong, period.

There are two TPS's, PF01 and PF09, the latter was used on the 4x4 (L8) and BT Escos.

The easiest way to determine which one you have (in lieu of being able to look at it) is to do a continuity test, PF01 is open circuit at idle, 09 won't be.

Martin