Why don't you try training in the 6+ rep range for a few months and then test the flip side of this theory?
If you come back and can't handle the really heavy weights for <6 anymore but you are using 80% + of your max for serious sets of 6+ then it would most likely be the adaptation.
But in the case of the other then you have found out that your biceps are predominantly fast twitch.
I kind of am... I'm now doing Animal Masses DFHT training (see sticky at the top of the board) and there are all sorts of different rep schemes...
If I was to do strictly 6+ reps on bis for the next 6 months it would be bad for a couple of reasons... Fast twitch fibres are larger than slow twitch fibres.. so If i eliminate a lot of my fast twitch work my arms will most likely grow less...
Also, it will totally screw up my new training since it is based on periodization...