Damage range

I was wondering if there is an explanation of the seemingly large damage range i take when a mob attacks me

for example when i fight the roadrunner's (dodo) near uldah they can hit me anywhere between 89 and 230, the SAME mob so this is not a rank issue

one mob hit me for 186 and then it "critted" me for 120... this doesnt make any sense

I have put more points into DEX and the average seems to be lower allthough its likely this is just a coincidence as im not sure dex has any effect at all.

any thoughts?