Do a search on "chloramine" to see a recent discussion in post titled "pre-algaed water" or something like that.
The tap water is treated with chloramine rather than just chlorine, that is what appears as CC on the test of tap water. It is more stable than chlorine for muni water needs. So it comes with a mix of ammonia and chlorine from the tap, as chloramine. Chem Geek said, if I understood correctly, that the chloramine will consume some FC to eliminate the ammonia part. So, if you added rather a lot of tap water compared to the pool size, you might see that effect.