Because it is a "heavy duty" fabricated deck as opposed to their standard deck. There are a few "light" fab decks out there, and JD makes a heavy duty stamped deck for some of their machines, so you really need to look at actual construction more than "key words".
The 48 might have a couple other perks too, but I'm not sure.
I didn't look close at it, but the ZF48 deck might have a smaller (or no need for one at all) reinforcing lip; if so you can get a closer trim than CC's stamped deck. Of course, if it is actually 2" smaller than the ZF50, that extra inch or two on the 50" would be nicer for getting under a hedge or something.
Anyway, weekly use on a small lawn, especially if it's cleaned regularly, probably doesn't require a heavy duty deck. If the only abuse you plan applies to the deck, then the 48 might be a perfect solution; if it's daily mowing, several acres, or severe abuse, then I'd be looking a couple more steps up.
Look 'em over, try 'em out, have some fun! Then enjoy your new machine! /forums/images/graemlins/cool.gif