Let's not forget that in some cases Klaus and Markus added a secondary "field quality" rating which was usually quite a bit (up to 2½٭) higher than the "technical" rating. It was usually reserved for fast primes that had some glaring deficiency when taken at face value. Sometimes there were even two such ratings added - one for landscape work and one for macro. Example:
http://opticallimits.com/nikon_ff/756-sigmaex2418fx?start=2
http://opticallimits.com/nikon_ff/756-sigmaex2418fx?start=2