But you can measure the cells or the energy used to fuel the car each time, no?

The fact that the manufacturers are subsidizing it makes me very suspicious of why they're providing it.

Speaking as someone who once worked on a H2 dispenser with a reasonably accurate meter, this is by the choice of the test program. Even without a meter you can figure out roughly how much H2 ends up in the car just by checking the tank pressure at the start and end of the fill. If it is a test program they are going to know the size of the tank (assuming that they don't have a working data link for some reason).