It's funny how the iPhone has been out years before the iPad but yet going with model numbers, there's just as many as the iPhone now.
Because the iPad hasn't had the "S" sub upgrades every other year. I suppose the current 4th Gen is the first one that could really be called the iPad 3S as it was a relatively minor tweak going to new dock connector and speedier processor, with everything else (size, screen, weight, basic design) being relatively the same. But it came on more a 6 month refresh cycle rather than the full year that has gone between iPhone upgrades.