Confusion about crochet hook size

[fatoldladyinpjs]

I bought a new Crystallite crochet hook in a size N. I got it home and the package said it was a 10 mm. However, my aluminum Boye hook is also an N but it's a 9 mm. My Boye is an old hook. What's going on here? Did the mm sizes change for this size over the years?