Pretty much, it's like taking an OS intended for use with only one system type and hoping it works with a completely new system the only way to do it properly with as little errors is to create a duplicate of the program but coded specifically for the new system its intended for allot of games for the ps3 that are made for the 360 also where made for the 360 first then a basis was introduced to use a small amount of effort to run it on the ps3 in hopes that it will run smoothly, when I got skyrim back in dec last year the only problem I saw was the fact the system was keeping useless data and piling it onto the current save file ie. files becoming 82mb when they should have stayed 3652kb it's mainly excess information that has no further use and instead of removing it, the system keeps it then piles it onto current information and that process is repeated several times until you start seeing obvious erroneous problems with the program itself ie. lag,freezing 100mb save files where it takes you 5 minutes to take 1 step.
Darksun its actually 7 vanilla experiences, if you count Morrowind,Tribunal and Bloodmoon for the xbox they had similar issues back then such as the "clean dirty disc" errors,etc.
Wow I cant believe you guys still haven't got Dawnguard or Hearthfire. Glad I got the 360 instead of the Playstation 3 even though, I started with the first playstation.