Relative and Specific Humidity Explained
If you look at the Humidity Spark Multiplier table in a stock truck calibration, you can see that have a dead zone where the multiplier is zero. Below that, the multiplier gets progressively negative (less total spark delivered to engine) in drier air or progressively positive (more spark added) in humid conditions. This follows the physics of what happens as more or less water is introduced into the combustion chamber during the burn. It's just physics, and the burn rate moves whether you zero out the Humidity spark tables or not. I would suggest leaving this alone and concentrating your tuning efforts elsewhere.