Frog Dragon

First, a little background info.
I have Big World installed, with most of the standard and tactics mods in place. I am playing Baldur's Gate 1 (on the BG 2 engine).

Now, I noticed that the archers aren't hitting the broad side of the barn regardless of their THAC0 and decided to toggle to-hit-rolls in the dialog panel. Here, I notice that in a lot of situations, even Kivan is firing on a negative attack bonus. However, this isn't happening all the time, sometimes the attack bonuses actually are positive.

I've noticed a few patterns with this.
1. When the target is in melee, huge attack penalties.
2. When the target is moving directly towards the shooters, no (large?) penalties. (Not a 100% sure about this one.)
3. In the start of combat, no (large?) penalties.

These are the ones I've managed to pick out. Now, I'm asking if this is a bug or a feature, and is it a part of the game or a part of one of the big world mods? (Checking the PDF of Big World, there doesn't seem to be anything, on a quick look that would cause this but I could be missing something.)