[Stephen Benskin]

I also assert that for Zone System testing, you also develop for less time. This moves the 0.1 density further to the right (slower).

ZS normal is around CI 0.58. Using a fixed density method for EI determination, the difference between the ISO contrast parameters and the Zone System method of determining contrast, the fixed density point will shift slightly to the left. Probably not a full Δ0.10 but contributing as Bill said. With the fractional gradient method / Delta-X criterion, there would be negligible difference in speed.

The range of EIs from most Zone System testing is generally from 1/2 to 1 stop slower than the ISO speed. I believe imprecision of using the camera's f/stop in ZS testing and experimental error are the major causes of the variance.