Firstly, there are now suitable products, endorsed by their manufacturers, that can be used on drying anhydrites and you should check the websites or call the helplines for further information.
However, I assume the issue is that the product instructions require the subfloor to be at or below 65% RH which is common for timber-based flooring and this is why you’ve contacted CFJ. The British Standard gives a rule of thumb for the drying of anhydrite screeds which is one day per mm up to 40 mm and two days per mm above this thickness (in ideal drying conditions), so it may only be a short time before dry readings are obtained.

The reason for the variability of the readings is probably due to temperature fluctuations as RH changes with temperature and hygrometers measure the airspace above the floor which will vary. The boxes may not be properly sealed to the floor. You could consider using a different method for measuring the moisture, such as the carbide bomb test which gives a measurement as a percentage of moisture to screed.
Richard Renouf, founder