+/- 20% is a large random variation range. Entity damage was change to only +/- 2% random variation in order to minimize the overlap between the quality levels.
And the effect gets larger as the Q level increases
20% of durability at Q1 is +/- 60 while 20% at Q6 is +/- 130. When the quality levels are only around 70 difference as you increase quality levels, you are going to see this occurring.
Range has the same +/- 20% variation in it.