In my 5th/6th split class last semester, I had a student who just couldn't be placed in any category. He had been tested for learning disabilities, but he had none. He typically did horrible in math, so he was with the low group and did the remedial work - yet he scored Proficient in math on his CST the year before. His verbal skills are far, far above average.
He is a rather explosive kid - quick to anger and quick to forgive. Quick to do anything - which gave him the talent of taking initiative, which is something many kids twice his age have yet to develop. He was also slow to think before acting - which put him in the middle of quite a few skirmishes with his peers (and arguments with his teachers). Still, most everyone liked him, and vice versa. I certainly enjoyed teaching him.
All the teaching and office staff called him a mystery because of his inconsistent academic performance (sometime really on top of it, sometimes not at all). During a grade level meeting with the principal, we were supposed to place our students into one of four groups: high achieving/high motivation, low achieving/high motivation, high achieving/low motivation, and low achieving/low motivation. It took me so long to finally decide where this kid fit on the spectrum. I've seen him act within the realms of all four areas.
Kids are kids, though, and they'll be inconsistent. That's to be expected. Certainly it's a challenge to figure out how to teach a kid who constantly moves around the board. But that makes teaching fun as well.