The data for the 2008 numbers in that link is messed up. It's in the form X/Y in most of them, but some have a single number, and a few are X/Y/Z. The poster says it is from different tests. It makes sense actually that most of the multiple scores are actually single tests with number correct/number answered. That is the only way to explain many of the scores. There has only been one guy to ever get a 50, but there are multiple guys with 50s there, so it is obviously not their score. 17/50 would be completely impossible if it were two scores, and it would have been pretty big news if anyone got a 50 or close to it, and a scandal if a guy went from 17 to 50.
From Wikipedia, here are average scores from other professions:
* Chemist - 31
* Programmer - 29
* Journalist - 26
* Sales - 24
* Bank teller - 22
* Clerical worker - 21
* Security guard - 17
* Warehouse - 15