No error, just not really relevant. Nothing in the game currently is affected by this fact, and may never be, but it's something I found interesting. The druid isn't trained in level 1 water spells, she's trained in level 1 of the water school. Not the same thing.
Basically, just pointing out that creature level and creature school are two separate stats. This creature is level 5, and arcane, and dark. Or pit another way, it is a leveln 5 arcane and dark creature. Or another, it is both a level 5 arcane creature and a level 5 dark creature.
Sorry for any confusion this may have caused. Absolutely no effects on game play and no need to worry about it, just me being geeky.