Thanks for the report I've actually found that the problem isn't related to the achievements themselves but the problem is that some of the numbers displayed in our profiles come from different sources than the one displayed under the achievements. Because of this it's possible to see these strange behaviours.
The reason why these sources of informations are different is mainly because it happens that someone try to crawl our website (that is, download all the wallpapers and stuff like that), so I have to try to restore those numbers to their values before the crawling, but during the process one of the sources of those numbers won't be modified so that soruce will also count the "fake" statistics coming from the crawling.
In short: the numbers displayed into the achievement page are the correct ones, and they are updated around once every 24h (I'll think about decreasing this delay). I'll fix the numbers displayed into our profiles, but for now part of our statistics will remain broken.
Looks like the problem that I've explained above was not related to your specific problem, you actually found a new unknown bug. It's fixed now, I'm sorry for the inconvenience.