I don't know why either, but every bow I've tried shot quieter split finger.
On the other hand, I've killed deer shooting 3 under, and I've killed deer shooting split. None of them complained about the noise.
I shoot split because I feel like I have better control that way, even though I can probably shoot better groups shooting three under. The thing is, as a hunter I shoot groups of one. I have more confidence that the one shot will hit the mark shooting split.
My son shoots three under and has no complaints about the noise. He shoots a Kanati, so quiet is kind of built in.
I'm not really sure one is actually LOUDER than the other. I think it may be more a matter of pitch change. I can't confirm this, but I have read of experiments using sophisticated equipment that showed little or no volume change between the two.
This topic seems to be one we worry over after hunting season, but in reality it makes little difference in the field. That's my two cents worth anyway. As they say, your mileage may vary.