I think there is an error in the interpretation of probe offsets within the probing module mcProbing.lua. The offsets XOffset and YOffset are defined during calibration as:

   Offset = Measured value - True value

(XPos and YPos are the variables representing  the true values as input by the user to the calibration function  Probing.XYOffsetCal.)  It therefore follows that these offsets should be subtracted from future probe measurements in order to achieve the desired correction to probe readings. In fact they are added, thereby increasing rather than minimising probing errors.

The solution is simply to replace all occurrences of the string "+ XOffset" by "- XOffset" and similarly for YOffset. In all there are 86 such occurrences, so you will want to use the Find and Replace tool in your editor to facilitate this.

Allan