Normally I am thinner on the top than the back

Regarding the HAIRCHECK test. It says that you take the permanent area as the base to compare. What if the top of an individual has always been thinner compared to the permanent areas? Then how reliable could you call the test?

I believe that people with fine hair often seem to be thinner on top, but it can be an illusion. I have measured many people with the HAIRCHECK instrument and those that are not balding always have the same readings on the instrument showing no balding present