There are several possible causes for variations between calibrations:
The surface was washed with a hot or cold solution prior to calibration, and was not allowed sufficient time to normalize
The plate is improperly supported (See Q.16)
Temperature change
Drafts
Direct sunlight or other radiant heat on the surface of the plate. Be sure that overhead lighting is not heating the surface
Variations in the vertical temperature gradient between winter and summer (If at all possible, know the vertical gradient temperature at the time the calibration is performed.)
Plate not allowed sufficient time to normalize after shipment
Improper use of inspection equipment or use of non-calibrated equipment
Surface change resulting from wear