CCs are constantly being formed so long as there is organic material in the water since CCs are simply chlorine combined with organics (or with ammonia). Additional chlorine breaks down CCs, but is relatively slow especially for some CCs while sunlight helps speed up the breakpoint process.
The 10x rule historically came from the stoichiometric relationship converted to ppm, but is based on Chlorine vs. Ammonia, NOT Chlorine vs. Combined Chlorine. The following is the best case set of reactions that occur:
Creation of Combined Chlorine:
HOCl + NH3 --> NH2Cl + H2O
Hypochlorous Acid + Ammonia --> Monochloramine + Water
Breakpoint of Combined Chlorine:
2NH2Cl + HOCl --> N2(g) + 3H+ + 3Cl- + H2O
Monochloramine + Hypochlorous Acid --> Nitrogen Gas + Hydrogen Ion + Chloride Ion + Water
So you can see that it takes 1 molecule of hypochlorous acid to form combined chlorine but 1/2 a molecule per molecule of combined chlorine to break it. So to go from ammonia to nitrogen gas takes 1.5 molecules of chlorine.
Ammonia is measured (by convention) as ppm Nitrogen with a molecular weight of 14.0067 while Hypochlorous Acid is measured (by convention) as ppm Chlorine Gas with a molecular weight of 70.906 so in ppm terms we it takes 1.5 * 70.906 / 14.0067 = 7.59 ppm of Chlorine to "break" 1 ppm of Ammonia. This 7.6 factor is where the 10x rule originally came from because this is a minimal factor with an optimum range being around 8-10.
There are two problems with taking this 10x rule to the chlorine tests of FC and CC. First, is that CC is already combined chlorine so two-thirds of the chlorine needed to get to breakpoint has already been used to form the combined chlorine in the first place. Second, the CC measurement is in the same units as FC, namely ppm Chlorine Gas, NOT ppm Nitrogen (this is because the test simply forces the attached chlorine to become free chlorine again by substituting iodine in its place and then the free chlorine gets measured). So there is no molecular weight difference.
This means that technically it takes a minimum FC of one-half the amount of CC to "break" it. So why the 10x rule? I believe the main reason is Cyanuric Acid (CYA). Though chlorine combines with ammonia very, very quickly (seconds or less) and combines with various organics at different rates (minutes to hours or not at all for some organics), the breakpoint reaction is rather slow even with no CYA where it's roughly a half hour to get near completion with 1 ppm FC and 0.1 ppm Ammonia (here, the 10x rule applies since we're starting with ammonia measured as ppm Nitrogen). In the presence of CYA, the hypochlorous acid level is significantly reduced and that reduces reaction rates proportionately. At 30 ppm CYA and 3 ppm FC, there is about 33x less hypochlorous acid so if one uses a 10x rule then this is 20x (due to the one-half requirement) the amount needed to break the chlorine, but mostly compensates for the 33x smaller amount that would affect the reaction rate. In other words, the 10x rule roughly gets one to similar half-hour reaction rates (without sunlight as a catalyst) for typical breakpoint without CYA.
If it truly took 10x of FC amount to break CCs, then one should see a HUGE reduction in FC when CCs got broken and that simply is not seen. Even with sunlight helping as a catalyst, it still takes chlorine to break CCs. The fact that CCs really only take half their amount in FC to get broken is why you never see a huge drop in FC when CCs drop.
Richard