[amsat-bb] Trying to understand downlink corrections

Greetings, I have been using a SDR dongle to receive the downlinks of SSB satellites. The combination of SatPC32's Downlink Correction feature and the SDR's visual display helps to zero beat the CW beacon exceptionally well.  The Lat/Lon is good to 6 decimals.  However, I find that the downlink correction needed is quite different for different satellites. Could it be due to rounding off errors or due to temperature variations at the satellite itself ?   Maybe this has been explained earlier, but I wonder why ?    Obviously, the more I learn, shows how less I know...  Thanks for any pointers !    73!  Umesh  k6vug