This might sound dumb, and maybe its a big series of bad luck, but:

I've fought these mobs over the course of numerous characters, and they wear a single piece of light armor, a belt, which does occasionally deflect blows.

I have *never* improved aim on these mobs.

You narrow your eyes and glare in a OneHeart seer's direction.
A OneHeart seer shakes her head in rebellion, shrugging off your divine power.
You have become better at light armor use (35%)! (100% learning bonus)

The only piece of light armor I've got on is a wool cloak with ebony clasp. Could be me getting it from that, but it seems unlikely.

Meanwhile fighting any mob with a shield makes aim fly up.

Could the aim code be messed up in regard to learning from mobs?