As soon as sample is added and mixed, molecules start landing on the glass slide. Variability in pipetting, mixing, and starting the recording will all contribute to variations in counts, even if starting with the same sample concentration. Most landing events occur right after adding sample and will decrease over the course of the measurement, so trying to keep an even pace (same number of pipette mixes up and down, same time between mixing, closing the lid and hitting record will all help keep your counts more reproducible between measurements.
As soon as sample is added and mixed, molecules start landing on the glass slide. Variability in pipetting, mixing, and starting the recording will all contribute to variations in counts, even if starting with the same sample concentration. Most landing events occur right after adding sample and will decrease over the course of the measurement, so trying to keep an even pace (same number of pipette mixes up and down, same time between mixing, closing the lid and hitting record will all help keep your counts more reproducible between measurements.