Using 10+modifier for opposed skill checks results in a bias in favor of the rolling party. Shouldn't it be 11 + modifier?
Example: a PC hides from a monster. The PC's stealth is +0, and monster's perception is +0. Even-Steven, right? No, the Perception DC is 10+0 = 10, meaning the PC successfully hides on a 10-20 roll (11 permutations, 55%), and is spotted on a 1-9 (9 permutations, 45%).
The same is true with opposed skills like deception and perception. The CRB tells a lying PC to roll Deception vs Perception DC, but if the PC is Sensing Motive they roll Perception vs Deception DC. If all modifiers are +0 again the PC has a 55% chance of success, so long as they are the one rolling. It's like the PC has a secret little boost during these NPC interactions.
If the rule was 11 + mod then equal modifiers would produce even odds. Wouldn't this have been a better way?