I'm not going to make that fighter roll for 20 feet, but 21 feet is by definition in doubt, or the rule would be, "You can automatically jump a distance equal to your strength score +1" or something. Now, it wouldn't be all that hard to go that extra foot, but it wouldn't be automatic.
And that's fine - heck, it's more generous than the rules technically allow.
The point (kind of in line with your previous post too) is that the fighter and the wizard need to exist on the same level of reality - not the fighter in "real world" reality and the wizard in "it's magic" mythic reality.