If they're getting paid 25%, then that explains how steam's able to run these sales with what appears to be such ease. Unless, of course, they had to prepare for half the year by getting permission to change the price temporarily from everyone.
Of course if getting 25% is what Valve gets for selling/distributing the games, cutting the price by 75% gives them no profit at all, so Valve probably just changed their mind (I expect it started selling even more like hotcakes) and brought it back up to 50% off (so as not to lose almost all the potential future profit).
I doubt the cut would be that large, and that's not exactly how it works. You assume the sales numbers with the event discount are equal to the sale numbers without the event discount, which they aren't.
Running a sale is trying to move volume of products by cutting profit margin and hoping consumers bite.
Say you have a warehouse full of masterwork turkey roasts. Each roast costs 3 dwarfbucks to make, and the vendor takes 10% commission on every sale, so 1 dwarfbuck. Selling them one at a time for 10 bucks nets you 6 bucks profit. If you sell 10 roasts at 10 bucks each in one week, that's 60 dorfbucks.
The next week you have a sale, 20% off and sell, 25 roasts at 8 bucks each. Suddenly you make 100 bucks profit.
Now imagine this with steam, where you don't even have production, transportation, or storage costs for solid media.