Sometimes it depends on the last of the boot / shoe that determines the size.
My ski boots are a size 25 (which I believe is a 5), the shoes I am wearing right now (Kickers - remember them from the old days) are a size 7 and most of the training shoes I wear are a 6 (25.5)
My ski boots fit like a glove though and last year I didn't need to take them off or loosen them to get them comfy - after all those problems I'd had getting them...