Relevant formulae:
A d100 Acc roll is made with a check of (Weapon Accuracy – Target Evasion).
Flying monsters have Evasion stats that are at least 15, maybe as high as 25, I would need to double-check.
Any ranged attack is granted an extra 10… or is it 15… accuracy versus flying creatures, negating most of this evasion bonus.
Immobilized targets are given a -10 evasion penalty.
Fun fact:
Flip a coin 100 times and there is a 10% chance that you will get a streak of 10 heads or 10 tails. Sometimes miss streaks like that just happen.