There can be a great amount of variation in the recorded Lux levels and/or daylight factors depending upon how the design sky is calculated and ambient daylight levels are handled. Refer to this post regarding the issue. Noting the following extract from Greg Ward:
The best thing to do is to use the -r (or -R) and -b (or -B) options of gensky to calibrate the absolute levels to match your weather data. Otherwise, gensky is going to use some default assumptions about atmospheric turbidity and mean solar brightness to come up with highly approximate brightness values.