PES is a name we use for PES Games.
WE is the name Japan uses for PES Games.
So a week after PES2010 Demo was released, the Japanese got thier version called WE2010. The PES and WE Games are the same, but the demos of the games are different.
Since the WE2010 Demo came out a week later than the PES2010 Demo, Konami decided to put a more recent 'code' for the WE2010 Demo. So basically the WE2010 is more up-to-date than the PES2010 Demo.
That is why a lot of people create Japanese accounts to download the WE2010 Demo. The WE2010 is closer to the real game that we will get.
So, if you want to have a go on the WE2010 Demo, simple create a Japanese PSN account.
Follow this link for some help to create a Japanese PSN account:
http://www.youtube.com/watch?v=d_Kfm0gCTX0
Hope this helps...