Hi Steve, the scanning equipment you use, is it MES?
If it is, it is very accurate via live feed.
If the connector block pins and gauge pod are in good condition and secure then there is nothing else much you can do... You have to remember that the gauge itself is just a service indicator and not as accurate as it should be really especially over time where loom,contacts and their relative resistance increase over the years.
Thus creating a varied reading compared to factory fresh wiring on the circuit.
I think it would be unwise to change the gauge pod for the sake of trying to rectify it, you may even find a reading drops even further.
It is an outside guess mind and worth a try for the cost is :
To replace the illumination bulbs in the pod itself... Sounds strange doesn't it?
There is a method to my madness
The bulb circuit on the rear is a read through circuit and the bulbs offer use as resistors even when the lights aren't on.
Its the same principle on the illumination circuit on the heater controls where a bulb out or giving low light causes many unusual fan issues...
Bulbs are pennies to replace mate.
Hope this helps