There can be organics to oxidize in the pool that do not register as CC in the test but create a chlorine demand so will use up FC over time. So if you measure significant CC or too much FC loss overnight then you have a need to keep the chlorine up at shock level. One example of organics that would not register as CC are those that are somewhat slow to react with chlorine, but once chlorine initially reacts with them then they decompose quickly.