Adding to what cofax7 said: trad climbing can be much more dangerous (not always, but it can be) than sport climbing, if the particular configuration of the rock on a route doesn't allow for much protection or reliable protection.

So you have the separate system of grading, which factors in how exposed/dangerous/"committing" a route is (if something's "committing", it means you can't safely downclimb out of a situation if you get stuck; the only way out is up), whereas sports climbs are graded purely on physical difficulty.