I can't even tell since when i'm experiencing this strange "behavior", i just never bothered with it because i quickly discovered a 10sp fix but i would be in trouble if i wasnt a cleric, i'll try to explain.
But first of all, yes i'm standing still.
I notice that sometimes for a reason i can't reproduce, my to-hit bonus(the one that we see when we click details in the inventory) becomes a lot lower than it should be. Reading the forum i've seen people claim it's just a screen bug and doesn't get calculated in real fight, but it does, mosters i should be hitting even with a roll of 2 only hits with a roll of 13.
My bonus which should be +28 standing still becomes +17 while this behavior is in effect, i'm a dwarf cleric level 15 with 21 strength(counting equipment and tomes), great axe proficiency feat, +2 hit with axes enhancement and +2 hit the spectral gloves give me.
As for the fix, simply cast a divine favor spell. To-hit goes up to +31 and after the spell effect wears off it goes back to the correct value, which is +28 at my level.