This is just a guess, but if your water temperature is colder upon opening, then if you do the CYA test and don't warm up the water first it will take longer to react to form a precipitate so will read artificially low. If your pool then warms up after opening, your warmer pool water would have a more accurate CYA test. In general, you want to have the test sample at least up to room temperature when doing the CYA test. Or you can just wait longer than 30 seconds for the mixing, though I don't know how long to wait -- maybe double the time for every 10ºF below 70ºF though that's just a guess (so that would be 2 minutes if the water is 50ºF).