Damned if I know. It's possibly the stupidest decision I've ever seen in a big name game. But yeah sometimes you'll be walking around and just all of a sudden get obliterated out of nowhere and it was because you got mapped by an NPC rocket with damage tied to frame rate. There's YouTube videos of people proving it works this way iirc; I know people used to post testing videos on R.

I don't know this specific case, but my guess is that damage is a set amount per frame that a collision overlap is happening. This is perfectly valid if it is biased by the delta between frames. It means things get a little wonky at low framerates, but largely work well. But if you are assuming a frame is an exact length, you save a multiplication action, but gain the problem of unlocked frame rates breaking things (and frame dipping possibly causing problems).