Sorry, I overlooked your Alk results both times.

This is a little above my pay grade but it might make sense for your additon of bleach to form a little CC....meaning there was a little something organic in there. I believe it is possible for CC to drift back down to 0 without shocking but I am not positive.

I test <.5CC frequently during the Summer and simply ignore it. If it goes to 1, I shock the pool (twice in two Summers).