Why have my Bitizen's skill levels changed?

In the beginning of the game I made sure to evict Bitizens with low skill level, and only kept 9's and 8's. Now I have noticed that most, if not all of my Bitizens have a decreased skill level with some even down to 0. Why is this happening?

Solution 1:

I know this isn't a definitive answer, but did it happen right after saving through Facebook? This happened to me, too, right after I signed in with Facebook. About half of my characters' uniforms got scrambled and most of them dropped skill levels only in their dream job category. I'm almost certain this is because of the different coding involved for different devices - Facebook saves have to be compatible with iphone, android, and windows OS's. On the bright side, though, you can get more Bux by replacing those workers with higher level bitizens, then rehiring the old ones to do it again indefinitely.