you can buy stack of 105 from AH and it will appear in your storage/inventory as stack of 105 even if you can stack only 5 .
this is for other players who have high stacking . if it wasn't like that some would have to trade to low lvl player in stack of 5 which would penalize players with high stacking trying to trade others with lower stacking.
the thing is you cannot stack it yourself neither can move / trade stack that high. and in order to auction it you need to move it to storage.
Same goes for stacking items in remote locations using trade/cancel method
Its been like that forever and dont worry about it
