Like you I can't be arsed enough to look for the exact statement (and it's probably in the old database, so it's pretty much as good as lost...), but I kind of remember it being something like : "we will try to make the best seller list good for one year" or something of the ilk.

Whatever the case, it's in fact total BS : Why ? The witcher 3 of course. It released around one year ago, so most of the copies it sold should still be accounted in its current total, and that still means that the d&d pack and the others have sold more than 500 000 copies just in the last year.
Hard to believe, huh ?
I'm absolutely certain that the best seller list is in fact worth till the beginning of the store, because there should be far more movement on it, if it were for only one year.

By November 11, 2011, The Witcher 2 had sold ~40k copies on GOG. http://www.pcgamer.com/gog-release-witcher-2-sales-stats-steam-dominates-all-competitors-combined/ - I think it's fair to say it's sold over 100k copies on GOG now (especially since it's been at such deep discounts).

We also know that Psychonauts has sold ~32k copies. https://en.wikipedia.org/wiki/Psychonauts#Sales

Not so surprising. First because many GOG customers (myself included) are nostalgic old geezers, who will avidly buy those games that they wanted to play back then but couldn't because they didn't have enough money. Nowadays, since we have the money, we buy them to abandon, even if we don't have the time to play (or the masochism to play again on some seriously dated UI)

Other reason is that most of thse games have been discounted or bundled to oblivion (especially the D&D games). Buying 1M games is less impressive when their price is only a few dozen cents.