I see no reason to think that people don't have different cognitive baselines. It's not difficult to imagine two people having such different cognitive baselines that even if exercise improves the cognitive ability of one, it will still be lower than the cognitive ability of the other without exercise.
I never did turn into a super genius after a run. But I can focus for a few hours and if it was an easy run or even a hard strength training session, it is a productive few hours :)
