I've noticed a similar issue with my Warlords often dealing 1 less damage than I expected, after manually recalculating bonus damage with their +30% boost. For example, at 5 base damage with 10% bonus damage, I expect to deal 7 damage: 5 + 5 * (10% + 30%) = 5 + 5 * 0.4 = 7. However, in practice a Warlord (and a Berserker, too, if I'm not mistaken) would only deal 6 damage.
After a bit of testing, I think I've figured out what's going on: Warlords (and Berserkers, too, I think) are having their bonus damage rounded down twice. Ordinarily the damage calculation is base damage * bonus damage, rounded down (so a character with 5 base damage and 10% bonus damage is rounded down from 5.5 to 5). However, the +30% bonus for Warlords is not simply added to their bonus damage, but rather calculated separately. In the above example, what happens is the Warlord does 5 + (5 * 0.1 = 0.5) + (5 * 0.3 = 1.5), with both of the bonuses being rounded down so the initial 10% bonus gets rounded down to 0 and the 30% bonus gets rounded down to 1.
I'm not sure if this is intentional or not; it's definitely not how it worked in the alpha - in the alpha, Warlords had their +30% added to bonus damage in the displayed calculation, while Berserkers got the full effect of theirs without rounding down twice. (Although Warlords no longer have their bonus included in the displayed value, they do have one advantage over their alpha counterparts in that the bonus is added if you're reduced to <50% HP after the enemy's first strike and applies to your first attack, whereas in the alpha the bonus doesn't kick in until after you've attacked.) I think I'd prefer the way it works in the alpha, but this shouldn't ever create more than a 1 damage discrepancy so I suppose it's not a big deal. It has been frustrating when I ended up leaving an enemy with 1 or 2 HP remaining instead of killing it as I had expected to, but now that I know the actual formula I won't make that mistake again; if it stays this way, it might be a good idea to have some clarification since I suspect most players won't figure out why they are doing less damage than they expected, at least not immediately.
EDIT: I just did a bit of testing with the Thief and it appears that his Stabber bonus also rounds down twice, although from a quick test in the latest alpha version it seems that he had the same issue back then... weird that no one seems to have noticed the discrepancy so far (especially since in the alpha he's the only one thus affected, as far as I can tell). Priests may or may not have the same issue; since a 100% bonus will never have a remainder I can't tell, and it's irrelevant for them anyway.