Does the level of the creature ITSELF reduce ActualChance to hit, apart from an expected increase in dex as creatures "level"? I think it does, but haven't really confirmed that.
Pretty sure it does, but no numbers to prove it.
Does offence rating scale with level as well as defence rating? That could be it.
The test would be trying the same fight at two different levels, without changing stats, and comparing your offence ratings. If offence rating doesn't change but % hit chance does, then it is a purely level-based thing.
I would prefer if it wasn't, as having things readable on the character sheet and less hidden from the player is more favourable for me personally.