Coincidentally, I also experienced a similar effect to the somewhat famous "two humped camel" programming aptitude test [1]. This paper argued that you can split candidates into two normal distributions based on aptitude. In my own tests, what I experienced was more of an extremely long tail on the right side, which is actually somewhat similar to the linked paper.

In our own tests, had a bunch of people fall into the 40-60% range, but there was a small bunch of people clustered around 100%, which are just people who could have scored 100%+ being binned down to the max score, and not a second normal distribution.

[1] http://www.eis.mdx.ac.uk/research/PhDArea/saeed/paper1.pdf