The CF_ATM and CF_1 values are calculated from the particle count data with a proprietary algorithm developed by the PMS5003 laser counter manufacturer, PlanTower. The specifics of the calculation are not available to the public (or us for that matter). However, to convert the particle count data (um/dl) to a mass concentration (ug/m3) they must use an average particle density. They do provide 2 different mass concentration conversion options; CF_1 uses the "average particle density" for indoor particulate matter and CF_ATM uses the "average particle density" for outdoor particulate matter. Depending on the density of the particles you are measuring the sensor could appear to read "high" or "low". Some groups have developed conversion factors to convert the data from the sensor to match the unique average particle density within their airshed.
https://www2.purpleair.com/community/faq#hc-what-is-the-difference-between-cf-1-and-cf-atm