Let me break it down. And I mean that honestly, because I really feel like this is a very simple concept, but apparently I'm not explaining it well.
Customer A uses Steam.
Customer B does not use Steam.
Scenario A: Valve leads off quarterly sales by being the first to market with their sales.
Customer A buys the game on Steam, Customer B does not see the sale on Steam, buys the game on Amazon, GMG, or another, and becomes a Steam user.
User Gained: 1
Sales Made: 1
Scenario B: Valve is the last to market with a quarterly sale.
Customer A buys the game he wants when he sees it on a steep sale on another site. Customer Be does the same, and becomes a Steam user.
User Gained: 1
Sales Made: 0
Does that make things clearer? Or are we going to again ignore this very simple concept and launch into another hyperbole driven rebuttal about how I supposedly think the sky is falling, Gaben is dying, and Steam is no longer economically viable.