Impossible Achievement is the one the devs use to test things. It is impossible for a standard player to achieve but can give glimpses of what may come when something has it as a prerequisite :)
Once the skill is ready for release to the public, all staff have to do is change the prerequisite and remove the Impossible Achievement from it, and bam, you can learn it.
Lol Doots! I can't w8 till they both come out either. The Specific achevement is kinda like the Buff "Overwhelmed By Devs" And someone else can explain that