The Sutro device has pins at the bottom of the device that push the reagents out of the cartridge during a reading. While your device is calibrating, it is using LEDs and other mechanisms in the device to position these pins correctly. There are a lot of things going on while your device is calibrating, which is why it can take a couple of hours after you first install a new cartridge. After calibration, it can take a few readings for the cartridge to stabilize as the Sutro gets to know your pool.