OTO is just measuring your TC. If the CYA is low then your FC will be burned off by sunlight. If you monitor the FC throughout the day you can see the effect. It is the FC that is the sanitizer. CC is more stable in sunlight and will not burn off as quickly but it is not an effective sanitizer. In fact, early attempts at stabilizing chlorine sometimes used anhydrous ammonia added to the pool to form monochloramine which is more stable than FC in sunlight!