I tried maximizing DR today, and noticed that it was recommending an off-hand that was not optimal. It should have been telling me to equip an old-school flying disc, which has 14 built-in DR because it's a shield, plus 10 DR as its enchantment (for a total of 24 DR). Upon further research, it appears that mafia thinks the old-school flying disc has a DR of only 10 (the enchantment). So, I'm guessing shield DRs aren't included at all.