Console games port

since all console games run on the same hardware than why do ports of console games have dramatically different requirements from the port of other console games? i mean that Test Drive Unlimited had different requirements, Pro Evolution Soccer 2008 had very decent requirements and Assasins Creed has outrageous requrements!!!
Like PES2008 runs at a smooth 60fps at 1280x1024 with maximum details on virually every 256MB graphics 7 series plus graphics card... ON the other hand Assassins Creed struggle?
why is this difference in requirements? i mean a small difference might be understandable but why this huge one?

 

1. Developers get better and better at optimizing content for a console. Thus later generation 360 games will look and play much better than earlier ones.

2. Porting is not a simple process. The more optimized a game is for a console, the more it's going to require from a computer.

3. There are bad ports as well. For example R6:V, where developers do a very shoddy job and rush it to the market just for sales.

 

Different (console) game engines will translate with varying efficiency. Some titles must be entirely rewritten when ported. Others (Bioshock and Gears of War come to mind) were written on engines that are already used on PCs.