It appears that the upgrade finder for warriors does not take into account impact of weapon skill on damage reduction for glancing blows on off-hand attacks. It does appear that weapon skill is taken into account for mainhand attacks.
I can’t tell if this means that glancing blows are not calculated as occurring on offhand hits, or the damage reduction is just not correctly modified by weapon skill.
Edit: Made it clearer what I think is happening.
Edit 2: Snapshot ID: be62ca3494f94f7587b77531c99cf19e