Yeah, I think a mistake that often occurs is thinking in logical terms. By that I means things like +10% smarts, must equal +11+% or else. Evolution is more random, one day an organism developes a weird modification, it may be a benefit, a problem, or not matter.
A super smart mouse might gain no advantage in normal tasks, but succeed wildly at say mating, or invent specialisation with its self as "High priest". Some selection is slow and steady, but plenty is wildly odd.