You mis read it. They are talking about all of PC games in general for that week and how Oblivion did vs those.
Todd's 10% is no way possible. Looking at TES NEXus downloads, its more like 25-30%
I think you misread my intention.
I was posting that quote from Bethesda's press release to show that Oblivion PC sales (at least during the first week of release) were abnormally high.
13% of all PC games sold during that week is enormous!
I would not be surprised if the total PC sales (including digital downloads, GOTY editions, etc.), are a significantly larger piece of the total pie than what the OP suggested.
However, since Todd Howard has stated that he believes "90% of [Bethesda's Skyrim audience] is on consoles," the Oblivion PC sales should not be too much higher than 10%, unless Todd himself doesn't know what the hell he's talking about. I think the PC sales for Skyrim will be higher, however, as cheap desktops and even some laptops are able to run the latest games these days at high resolution.
http://www.escapistmagazine.com/news/view/109378-Bethesda-Wants-Skyrim-Mods-on-Consoles