By playing an arena game with /maxcarry 6, then picking up a grenade and mine, weapon slots 1-9 will all be full (and 0 is reserved for melee which isn't used, so the key for /weapon 0 has no effect). Proceeding to pick up a rocket will fill a 10th weapon slot, which displays a shortcut bind of "10". Needless to say, no shortcut key will match that, though selecting it directly with /weapon 10, as well as mouse wheel scrolling, both allow access to it. So from a navigational standpoint there's no real loss, just that "10" doesn't gain anything as a shortcut "key" either.
You could also edit a config file and add a bind that specifically calls out [weapon 10], which also works, and it displays the more sensible shortcut going forward instead of 10. However, the options screen doesn't give a way to set that bind--it still harkens back to earlier days when rocket counted as weapon 9, and mines didn't exist (or take up a slot) at all.