Ever since one of our rogues blocked a melee attack in Zul'Gurub (partially blocked 7 damage, fits as it's the block value you gain from 140-160 strength, a typical ZG level rogue value), I just accepted that sometimes glitches happen that shouldn't happen nor can be explained.
Did that happen several times, or was that just a one-time incident? If so, it might have been a glitch like that.
(No, I don't have a link to the screenie of the block anymore. And I can't force you to believe things you don't want to believe anyway

)