Is there any particular reason why an elven (or even human) character with extremely good agility should be consistently poor at darts?
There's a coded reason why they would be. Is your question about that, or more about the rationality of not making agility count for more in darts?
Using common sense, I'd imagine the "throw" skill would help out in this.
Same reason a character with AI strength might still suck at Giant's Fist.
But this purely speculation.
I could guess at more reasons than agility for accuracy in darts. skill_throw being one.