This is because microsoft payed them.
Don′t act like this is a "pre test for the pc and ps3 release".
Reason why it is always xbox in the end of many ads is because Microsoft payed them , most games have like the pc format, ps network and the xbox signs on end of the games.
High priority releases often get payed by microsoft just to make the ad look like its xbox only.
The ps3 format might be smaller but its not lesser then 50% smaller ATLEAST.
A theory:
Thing is that the americans are many like we all know and they tend to buy xbox since its microsoft.
meanwhile asia buys ps3 since its sony.
Even though both countries buy each other console its just people thing about "supporting there country"
Meanwhile Europe just buys whatever they feel like buying since they are not supporting any company.
I do not have any experience outside of Northen Europe .
i live in Norway where everyone has a ps3.
So i guess its like that in danmark and Sweden.
I lived in Iceland where it was just like that if you would take a look at the Icelandic community like psx.is and xbox360.is you would see that psx.is has almost triple the members of what xbox has.
I think this is because people are more spoiled and there for buy the ps3 and they have played the ps2 alot and do not want to change. Sidenote most people i know [censored] on the xbox , think haha you are gonna buy a xbox to do what use it as a doorstop ?.
I don′t think asia purchases as much as america does (remember you guys are in the most debt americans and even though Iceland owes much in terms of the population, you guys owe thousand times more and there is a stupid reason why Iceland was called bankrupt, Even though it wasnt quite bankrupt just you powerfull countries decided it was bankrupt and then it collapsed right after you guys decided it, we even got less people without a work then spain,portugal,america,france whatever you name it! yeh im pretty pissed because of this , because i currently live in Norway since some stupid a hole decided to create ice save then move to UK lol! well lets get off this sidenote)
Anyways, Microsoft has endless of money and companies like to make money that is why i don′t blame Bethesda for this, all companies sell out activision did , Rockstar with GTA series,Bethesda cant remember more on the top of my head but let′s face it, THIS IS Microsoft!
Backstory:
I own two ps3, gaming pc and i will be getting a xbox for xmas so i am not a really devoted fan to any system , i hate microsoft btw because they bought out cod my fav game and they bought out fallout my fav RPG game :C
Bottom line: Microsoft pays. (hope this wasn′t way to confusing, please don′t scream back at me , i hate bad comments against me no JOKE!

EDIT: yes this bugs me , like you can see.