The biggest problem is that when Apple updates iOS, it takes the users which represent the majority of the iOS installed base at least 3 months or longer until they update an existing piece of hardware. BUT, some models, won't get updated. EVER. So if you bought a 2.X phone, you may not get 6.0 If you got a 3.x phone you won't get updated to 7.0
Fixed that for you.
Also, 4.1/4.2/4.3 of Jellybean are minor feature release numbers, there are patches being applied to 4.1.X for security reasons which most OEMs can roll-out pretty quickly, even if they haven't moved that handset over to 4.2 or 4.3.