Generally, the rational I've seen for using higher reps for the lower body is that the muscles there are of greater size and work capacity. I think what's more important is experimenting to find what's best for you. Depending on the predominating fiber type in you're individual muscles, some upper body muscles may respond best to a long TUL and some lower body muscles respond best to a short TUL and vice versa. I've found that I, like many, do best on lower reps, i.e. 3-5. But for someone with a predominance of slow-twitch fibers this would be less than ideal. Occasionally, I'll use higher reps, for instance if I find my form is breaking down. But in general I find that very high reps are more a metabolic/mental exercise than anything.