They do some adjustment from time to time, you can see that 化ける was changed recently here:
Now I think it’s cleared that 化ける is mainly used in the sense transform oneself, turn oneself into. You can check the example sentences to have some idea.
表 has quite a lot of meaning. You can check on kanjipedia that “express” is one of them (meaning 2). I think they chose this one because almost all the vocab taught by WK involve expression/express. But the meaning of surface is also important (and it’s probably the original root meaning of the kanji), and is taught as a vocab.