That's actually not an offsetting factor at all if you try to gauge your own strength. It only creates a percentual discrepancy towards other players, and only if they are using multi-hit WS.
Another factor that's more bothersome is the distance. You only receive log information from players within 30' of your character. So if there's two mobs being fought a bit apart, you miss out on that already.
Regain is another thing, since it builds your "damage" even while you're technically not engaged. Also good damage dealers slowing themselves down, by being target of the enemy, getting TP/food stolen from colibris, or getting stunned/terrorized, and other fun things like that. Fast-hitting jobs are more bothered by spikes as well, are, on average, longer exposed to paralyze and stun, etc. There's plenty of inaccuries, whether gameplay, technology or parser related. The best estimate you can get is by theory. Fortunately our current understanding of game-math has progressed fairly far.