[Incoherent]

Bill
I think there is a measurement error.
It is not necessarily an offset, or at least the offset could be proportional to T.
I note "*RTD calibration in hardware". What kind of RTDs are they? PT100, Ni100, Ni120, CU10 etc? they have different temperature coefficients, even within the same type (i.e. PT100s can have different coefficients depending on the standard adherred to). Could it be that you are using an alpha value for a different type? This would defy calibration of the RTDs against a "known calibration of one", or comparing at zero load, they would read the same at the same temperature, the error coming when they have a difference...
(Edit. Some misleadings statements here. The "known calibration" would work, if it is itself correct. The effect is a compression/dilation otherwise)

... is pHaestus using RTDs???


Using a PT100 RTD with a NI110 setting for example might give this error within the small delta T we are dealing with.

Sorry if this is way off the mark, clutching at straws really, I just do not like these curves.