*Evil glare*

You've just got a dud Black Belt. It can happen to any class. For most level ups, there's only a 50% chance a certain stat will go up. It's just that your Black Belt hasn't been hitting that sweet spot.

At level 20... A White Wizard can have a maximum of 280 HP. And a Master can have a minimum of 215.

Just like at about any given level, your Black Wizard could tie your Knight in terms of STR. It's all just a crapshoot.