It has to do with temperature and alcohol content. A big beer needs more time to carb and beer generally takes longer to carb during winter than summer (at least here in the south where room temp during summer is 80+ whereas it's closer to 68 during winter).
Edit: I was considering saying something about the minimum of three weeks thing as a joke... guess I was beaten.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Primary: Saison, Berliner Weisse, sLambic I, sLambic II, IPA, 70/-
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong, Dark Mild