Put a jumper between ground and the signal pin on the input after the one you're reading. If you're reading input 5, ground out 6.
The inputs are all read by the same analog to digital converter, sequentially. The inputs by themselves have no resistance to drain off voltage, so the voltage from one input will carry over to the next input as they get read.
It really shouldn't matter though because if you don't have a sensor attached to those inputs, you probably don't care what value they are at. In the test mode when you can see all input values, then you will see the voltage carry over.