I did some research on this about 6 months ago. I believe this is an evolution of the charts meant to indicate what current is flowing through a specific fuse size by measuring voltage drop across the fuse. I found that there were two main alloys used in the fuse metal strip; they were very different in the resistance values. This means different voltage drops per type of fuse metal for the same rating. The other important factor was that fuse resistance varied 1% per degree centigrade. Fuses are rated at 68 deg F. Bringing a car into the shop on a zero degree day is going to mean a 15% change in resistance (and thus error) if you start to diagnose it right away.

Matt De Maria
Owner/Technician
Stuttgart Performance Engineering
Gaithersburg, Maryland, USA