foxworks: Probably due to the limited quantities ;-)
Niggles: Im actually wondering if Xmas sale prices are usually the best compared to GOG sales for the rest of the year.....
They're pretty much the same at all of the various sales throughout the year. 50% discount on a large number of titles, often most of the catalogue, combined with daily and/or hourly deals of 60-90% off on either individual titles or collections of titles. The majority tend to be about 75% off, with a much smaller number at 80/85% off and rarely an occasional title for 90% off. As time goes on, the older a title gets their base price goes down also, and the amount of discount offered on the title may go up during promos as well, so future promos will see higher priced games gradually become increasingly cheaper, until they plateau at the "long haul" price where they'll sit likely indefinitely.
My favourite promos are the summer and winter sales personally as they offer a lot of great deals without too much time consumption. This particular sale is very nice though even though there is a little time consumption tracking the flash sales, but since they'll be repeated many times it seems, one doesn't have to be glued to the website either. :)