How about this? It's only a theory but it makes some sense to me.
High mag = lots of more wobble. Your brain will tell you to release the shot the instant your eyes tell it you are right on target. Result more of a snatch on the trigger as you try to fire at that precise moment (even though you know you shouldn't) Snatched trigger = larger groups.
Lower mag = a lot less wobble. Less presure on your brian to say NOW the second your eyes tell it you are on target. So a much smoother release and a tighter group.
I also find lower mags give better groups. Although at anything beyound 50 yrds I don't think less than 12 x gives enough precision.