I assume that it comes down to the same USn/Can. v the rest divide??

Careful there, johnjohn! When was the last time you heard a USn saying "leff-tenant"? We don't pronounce everything like them just because they're big and sell us a lot of movies!

Herbs [charades-sounds-like-emoticon] "erbs". But it regains the "h" when I say "herbal".