They people who made lumineon probably designed him in a way to make him special.
Erratic means un-regular or unpredictable. This means that lumineon earns various exp. each Pokemon or stage. It has no desireable pattern.
The reason luvdisc says "fast" under growth rate, it means that it goes under a consistent pattern. It distributes a higher percentage for each Pokemon, under a constant pattern.
So, lumineon may be different than other Pokemon, but this doesn't me it's not worth keeping or training. Just continue with your lumineon - no worries.
Hopefully, I've helped!