Through observation and empirical evidence, a lot of strength trainers and bodybuilders have found that 5-6 reps is ideal for building strength, while 10-12 reps is ideal for building size.
I've noticed in my own experience that it's a lot easier for me to add 10lbs to a lift at the 5 rep range, than it is for me to keep the weight for my previous 5RM and go up to 8 or 10 reps.
An obvious answer might be that increasing the reps significantly increases the total load (8 reps at 100lbs is 800lbs total, while 5 reps at 110lbs is 550lbs total), but I'm wondering if there's also something else. Football players are tested for the 40 yard dash because that's about the distance that can be maintained at top speed, around the 5-7 second range, while a 100 meter sprint would have even the best sprinters slowing down slightly at the finish compared to their top speed at any point on the stretch.
Is there something similar going on with lifts? Such as muscle glycogen stores being exhausted in the 5-6 rep range, and the body having to switch to breaking down lactate for energy in the 10-12 rep range?