How to correct/adjust floor temperature sensor resistance to account for calibration error?
7°C is a huge error- typically sensors like that are good to a fraction of a degree C (better than most semiconductor temperature sensors). 

The 10K resistance is typically specified at 25°C. If  you have a "Beta" spec you can look up the temperature-resistance chart and find the expected resistance at 21°C. For example a beta 25/50 of 3380K (a common type) would have a resistance of 10K at 25°C and about 11.67K at 21°C. 

Once you know the expected temperature-resistance curve you can determine where the error lies. My bet would be with the controller, however such a huge error (reading resistance of about 8.9K in this case rather than 11.7K) would imply either something is seriously wrong with the controller or there is some configuration setting that you need to make. Maybe look for a service manual for the controller or contact the supplier. 

Just to be absolutely clear on this- with such a large error (assuming no hidden configuration setting you've possibly missed), there's a good chance it's about to fail completely and without further notice.