Oh dear.
The AMD socket ignorance too.
Not sure how it's marketing greed (1 for your mainstream, one for your enthusiasts), AMD have two sockets on the go too (FM1 due to be replaced with FM2 and AM3+ which will probably be replaced after Piledriver)
I'm sure Bulldozer was touted AM3, works on AM3 but only a handful of boards ever got updated to Bulldozer (3 as far as I'm aware), instead you're forced to buy AM3+.
There's other instances as well, but AMD certainly don't keep the same socket always, least when Intel do it you get quite a performance boost
AM2+ to AM3, both took the same CPU's (Phenom II/Athlon II & Minus the 3 on Bulldozer. That's a socket & RAM change without any actual improvement except in memory performance and the native sata.
Obviously, the fact Phenom II worked on AM2+ is good (And a handful of AM2 boards), but AM3 didn't officially get a new CPU, yet it was a new socket.
PD launching on AM3+ is another good but, but Intel have already launched 2 CPU series (SB & IB on 1155)
But if you look at the figures, AM2+ to AM3+ (3 sockets in total) what performance increase has their been from what the lowest socket can take (Thubans)? Not much, IPC has gone down, only thing that's higher is multitasking but that takes an FX8 and isn't a consistent performer.