I have been wondering about this too, as I had started playing the game a few days ago, and I don't see anywhere in the Survivor's info about what Skill level 1 through 5 they are.
From what forum posts I have seen about game updates and changes, it looks to me like that might actually be one of the changes, and the 1 through 5 are not used anymore.
If that is the case, then the related game Achievements may have become obsolete and unattainable.