I think it's just that J, K, L, N, and X don't mean anything to the uninitiated. They're probably just trying to make their website more easy to follow for the general public.
For those of us who are initiated we know what those letters mean and most of us can tell what frame it is just by the model number. I even find myself picking up series of numbers in everyday life and thinking about a particular model. Like when the cashier says "that will be $5.47 sir" and I think to myself yeah I'll find one some day and I hope is a 3" RB!