To high values ​​in the measurement of cement screed

Bought to measure the moisture of our new screed the device. The values ​​were among the first measurements approximately 14-15% (of the screed is already for 6 weeks and the professional had measured already after 3 weeks 8-9%; Schnellaustrockner in screed) which seemed to me very high. One week later, the device showed more than 12%. The expensive device displays compared plausible at 1-2%.
Conclusion: device does not work for my application. It gives more concerned, because the values ​​are too high.

Note: The Brennenstuhl device was set to "building materials". The more expensive device has a mode for "cement screed" and had set it.