Although there are literally hundreds of poker websites out there, there are only around 12 gaming softwares that run these accounts. For example as you have found out, noble and Cd poker run the same software called i poker network, they also share with USA poker,BET fred, Diamond club poker, Poker 770 and a couple of others i think. Royal Vegas
and Crazy Vegas and all other prima sites are run by microgaming software.
They do this to maximise the number of people playing on the software, although each poker site
is competing against the next to get the customers. Party Poker
shares the biggest network but off the top of my head i can't name it although i know empire use it aswell and they continually have in excess of 25,000 players on.
You can register at each site if they use the same software but make sure you use the same details otherwise they will shut you down to prevent collusion. like i said, each site wants its own customers to deposit on their site even though they share the same rooms etc. You cannot log into two sites at once that run the same software if your at one computer.
Hope this clears things up for you.