For what it's worth, this also happens when grading essays.
In any batch of say 30 students, I'd expect most of the grades (16-17) to be in the 66-68 range, with a few 70+ (let's say 4-5 firsts) and a few in the low 60s (4-5) and 50 range (4-5) and maybe one in the 40s.
I'd be interested to see if such "bunching" occurs in other areas. I used to read a lot of music mags and A LOT of albums would get 4 stars. 4 stars is a safe "This was really good but we're not going over the top" sort of rating.
I used to read a lot of video game mags too. There 90+ was your classic 5-star, and you'd get an awful lot of games in the 80-89 bracket.
It happens everywhere I reckon. Things bunch in the "very good but not excellent" bracket.