The only people I ever saw where the sizes didn't match, the forearm was longer than the foot... But that is like, 10% of ppl. In my experience, most of them match.You rely on the myth that foot and forearm are the same length. Let me emphasize the word myth. Since that's all it is. Thank you very much Da Vinci.
Of course there is a correlation between both lengths and for some people they are the same length. But you can't assume in general that they are the same since we're not vitruvian after all.