Glancing blows are common with two handed weapons. They should register as separate from doublestrike chances. To test it, loiter around a weaker enemy, stay stationary and fight and watch that Combat log.
My guess is this is what you are seeing.
Which is additional DPS, not less. (or supposed to be anyway... I have never done the math)
Originally Posted by Jandric
..., but I honestly think the solution is to group with less whiny people.