Fog in satellite images

Winter last year i stumbled over a satellite image product at I don’t know how the cloud mask on this site is created but it seemed to have a problem with fog. Looking at this image only one gets the impression that there are absolutely no clouds over Germany. Looking into the sky proves that it is foggy. So this cloud mask could be a little bit misleading when used for example for estimating temperatures.

But how can this difference of observation and product be explained?