Nasus E armour reduction vs jungle monsters.

Played a coop vs ai game as jungle nasus, noticed like 6+ minutes in that the jungle mobs seemed to be having their armour reduced by double the amount it should be. As the game progressed, it because really inconsistent. At first the enemy jungle mobs weren't having this happen to them, but then later on it started doubling; after I noticed that, I noticed it was TRIPLED against the home jungle mobs.

Is this some sort of hidden mechanic? Or maybe some value is stacking up weirdly because of how they respawn? I dunno, but it seems like it's not supposed to happen.