Will try and find some info for you.
The thing to remember is that it is a totally different system to the "Traditional" method which people have burned into their brains, this is why they work soo differently.
First of all, there are "Unlimited" copies on STEAM, unlike traditional retail where you are governed by the number of "units" you have.
There are no "tangible" assets for Valve to distribute so how would it work, do Valve buy "10,000" downloads from the developer at a certain amount? Of course not, that would be silly.
The developer gets their money as soon as their game is sold, Valve obviously take a percentage as their cut, that is how they make their money.
This gives the Developer/Publisher MUCH MORE CONTROL over their product compared to retail.
If they want, they can even (For a fee I assume) give their COMPLETE game away for a weekend if they want, VALVE can not say "Hey, lets give someone elses work away for free".
If an item is not selling too well for a publisher or they have released a MAJOR patch which fixes game breaking bugs and they want people to try the game, half the price for a week, why not?
With the traditional retail method, this cannot be done as the retailers have allready paid for the stock and the publisher cannot tell ALL their retailers, "Hey, forget about your profit margins, half the price of this game", the stores cannot as they have their own overheads to consider, plus has mantioned, they have allready paid upfront for the item.