If I remember my A-level Psychology correctly, some older IQ scales capped out at a lower number than the current one. Maybe it's a conversion to the French system of the time?
Alternatively, they decided 85 wasn't low enough to convey stupidity, since its only really a step below average IQ.
Or maybe a typo by the translator, who knows? In the French version of Transformers, Starscream was a woman so it's hardly the weirdest example of lost in translation out there.