The easiest way to look at it is that the industry has never developed the appropriate chemical test to determine the true amount of disinfecting and oxidizing chlorine (that is, hypochlorous acid). The closest tests are expensive sensors with selective membranes that measure hypochlorous acid level and ORP sensors that are a rough proxy for it (though they vary by sensor manufacturer).

The Free Chlorine (FC) level is a simple chemical test, but it really just measures the total reserve of chlorine so will tell you whether you have enough total quantity to not run out if there is a certain rate of consumption. It does directly not tell you the rate of disinfection, algae kill, or oxidation.

Fortunately, the chemistry of chlorine, Cyanuric Acid, and their combinations is well understood (has been since at least 1974) so one can use the FC/CYA ratio (or FC percentage of CYA) as a reasonable estimate of the hypochlorous acid concentration. Though this ratio relationship varies somewhat with pH, it only varies by about 25% between a pH of 7.2 and 7.8.

Richard