No, I was wondering how they calculate the percentages. It looks like they count only about 90k Glitchen, while it seems like there are a bit more in the leaderboards. Do they count everyone with one achievement or what?
It is based on how many people have that particular achievement as Schwm said. It isn't a percentage of the Glitch population as a whole from what I understand. I think I'm not wording this right. I need something to eat first.
Glitchium looks at players who are brought to its attention, by someone (whether the player themselves or some curious third party) entering their name into one of their search boxes. They may have initially seeded their data in some other way, I don't know.
The data came from the official total counts, which were displayed on the "[Player name] has earned the ____ achievement" pages. Unfortunately those pages actually disappeared shortly after the Glitch closing announcement so now there is no way to obtain official counts now (the pages also had the magic rock quotes when you earned a badge). Those pages never showed hidden achievements so counts for hidden achievements are based on only a partial sample of scanned players. The total count is based on the leaderboards count for people with at least 1 achievement.