Percentage doesn't decrease if new achievements are added

[RandomArchI]

So, I've had this problem with a number of games already. If the developers decide to add a bunch of new achievements to a 100% (on my side) game, the site doesn't change "Achievement percentage" even if a change is very significant (down to 58%, for example). Not sure about the reaction for a non-100% though. One of the (more or less) recent examples is "Boid".