As I said, the value for 'free T' you get is a test number specific for THAT test and does NOT give the absolute biological amount of free T. It is just some number from some scale that shows if you are normal or high/low with respect to the sample of people that was used to create the lab test.
It is completely logical and actually consistent that the top of the normal range for Quest, 15.5ng/dL should correspond to the top of normal range for Lab Corp 25 ng/dL if they used the same statistical definitions of 'normal' and had defining samples of similar means and standard deviations.
The problem arises if you actually take these numbers as the actual value of free testosterone. The number on a test only gives the relative position within that test scale. Online calculators for free T will give completely different values within their own scales.
