Yang has awkward HP growth from a result of a cap placed upon him in the original Super Nintendo version of the game. On the North American and Japanese Gameboy Advance versions, this cap was not removed and, as a result, his HP growth continued to be capped until the cap was removed in the European release of the Gameboy Advance version.

I assume that the cap was placed upon him because his HP grows exponentially at certain points. Although he wouldn't be in the party at higher levels in the original version, I suppose it was placed upon him so that he wouldn't hit the maximum HP level way before a player was intended to see it, or something...