I was definitely surprised. Yes, the game had more than 20,000 votes on the wishlist, but:...
There were ads for it on a couple of sites, I think RPS had them, so that must have brought a bunch of people who were new to the site and many of those who may have forgotten about GOG or voted for SS2 a long time ago. Also, not everyone uses the wishlist, so one can only assume that the 'wishes' on the list is only a fraction of the people willing to pay for the game. I'm not surprised in the least if the number of buyers surpasses the wishes on the list for many of the games that are finally taken off the list when GOG manages to put them on sale.
Well, yes, that's kind of what I said: The on- and offsite marketing more than compensated for the (expected) fact that not everyone who ever voted for the game on the wishlist will be present, able, and willing, to spend money on the game right away.
I'm not surprised that these effects exist, though I _am_ surprised that the former outweigh the latter for a game that had such a high visibility on the wishlist for such a long time. I'm happy that I was wrong though. :)