Some of this is likely wrong, but I'd say it seems about spot on that COD BLOPS total PC sales are around 2 million units (being Steam only represents 70% of the DD market on PC according to the most recent analysis info, and counting retail). The games after that I do find questionable in some instances.
I love Steam, but we have to consider that recently Magicka made it to the number 1 selling game on the service its' first day while only pulling in ~30K. I think Steam is big, but I question whether it's the juggernaut that many propose it to be.
Magicka was released on a weekday. It's on weekends that more people play, buy and are logged on Steam. There's also over 1000 games being sold on Steam, and if 1 game alone can make $250k a day, that bodes really well for Steam.
Also, PC gamers aren't as frontloaded in buying as console gamers, which means that PC games keep selling for years, no matter how slow. You think people have stopped buying Garry's Mod because it's not even on Steam's Top 20 sellers?! Wrong: http://news.bigdownload.com/2011/01/23/garrys-mod-sales-continue-to-increase-year-over-year/
"However the mod continues to sell well as posted by Newman himself in a recent blog post. Back in late October he admitted the game has sold over 770,000 copies since it was first released. In his new blog post Newman states that sales of the game went up by 30 percent in 2008 following the game's first full year of sales in 2007. In 2009 sales increased again by 31 percent and in 2010 sales went up yet again by 33 percent. Yes, that means that despite being available for four years sales of Garry's Mod have increased nearly 100 percent since it was first launched."
Garry's Mod sales kept increasing every year, even though it continued to decline on Steam's Top charts.