Bear in mind that different brands size differently, too - in the same way as for clothes, the cut seems to matter. My lady recently bought a pair of Art boots (OMG, totally gorgeous!) that are the same size as her boots from Blowfish, but the former are noticably smaller than the latter.
It's not a matter of inches but it's enough that she had to go up a size in the Art pair!
Surprising, but true.