Cos this stuff comes up in comments most every time I run one of these: these charts depict the top ten best-selling games on Steam as accumulated over the week leading up to Sunday just gone. They are not what are the top ten best-selling games at this moment in time, as seen on the front-page of Steam and which are invariably a little different. They come from this here Valve RSS feed. If there is any massaging of figures or weighing of e.g. revenue earned vs copies sold then I do not know of it, but neither can I say for certain that there is not. This is, however, pretty much all that Steam ever lets slip about what’s going on, though you can look to the guesstimates on Steam Spy if you want to try and drill down further into actual figures.
So: Steam’s ten biggest games last week. Well, nine and a half. Deus Ex has been dethroned already.