I have noticed those price differences on Steam a while ago. They most commonly convert dollars directly to euros. Well, that's not a conversion. They just swap out the currency symbol. That's pretty damn uncool if you ask me, because all tax included, the prices do vary greatly.
But hey, who wants to buy Valve's DRM crap when you can get yourself a nice disc filled with goodies directly at Telltale? I never liked Steam and I only used it because I had to [to play HL series and other Valve games]. Not too long ago I have finally decided to uninstall Steam completely, because it will take a while before HL2: Episode 3 comes out and I've played quite enough of The Orange Box and Counter Strike.
In other words ... Forget about Steam and get yourself the real deal at one of Telltale's numerous discounts. This way you won't have to put up with all the DRM and you won't contribute to Newell
's hot-dog fund.