Hmmm. I'll have to test a bit; I don't often use darts, so I haven't noticed that effect. On second thought, I have thought that thrown weapons in IWD don't do as much damage as they should (primarily axe). I wonder if the Str bonus is not added to thrown weapons?
I'm confused.
I do know that the 2nd ed. PnP rules call for the addition of str and dex modifiers to thrown weapons. I can't say for certain that IWD uses the exact same formula.