Humidity in the air is your likely culprit that varies. You can boil off more wort on a slight boil in winter than you can on a vigorous rolling boil in the summer because of differences in humidity. I would put a post boil mark in your kettle to also know where you stand (I mark my mash paddle).
Also as a liquid decreases in temperature (while you are chilling), so does your volume. Ever notice why when you fill a carboy up with oxy and hot water you have to keep adding water to keep it topped off? Its becaues the water is cooling and occupying less volume.
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