Maybe this will help.
Zone System vs ISO.jpg
What is basically happening is the Zone System test changes the ratio between the speed point and the metered exposure from the ISO's Δ1.00 (10x) to the Zone Systems Δ1.20 (16x). It isn't so much about determining film speed.
The examples deal with exposure in testing and not exposure placement in use. Flare still has to be added to the mix.