The foot can change shape and size as you get older. Also there are many different "lasts" (that wooden foot shaped model used to build a shoe around it) and they all have a different volume on any given size.
See here for details :
http://www.footjoy.com.au/customerservice/shoes_fitting.asp
So when a brand switches the order to a different factory you can end up with a different sized shoe . That is why I would not buy a shoe on the Net, I still occasionally get it wrong AFTER I have tried them on.
Franco