Thanks the attachment. I think I understand most of it. I deduce 2nd column is your MIG, 3rd column is external lab instrument.
If yr tolerance of +/-0.5 degC applies to the accuracy of the indicated MIG temperature in the range shown on the certificate, the MIG looks to be out of tolerance since the errors vary from 0.6 to 1.8 degC.
I have typically seen tolerances used from +/- (0.5 – 1.0) degC but the actual choice may depend on the specific use of the MIG and sometimes even Regulatory requirements.
Internal checks are usually limited to testing at 0 and 100degC as you no doubt know. IMEX the results are always a bit different to an external lab since the latter should have a more controlled environment and standardised testing conditions (in theory anyway).
My experience is mainly with thermocouples where you can adjust the zero to electronically correct for the error at a specific temperature and at the same time reduce the errors at other temperatures. Is it possible to adjust the scale on the MIG at all?
I suggest you have a look at thread below which has some similarities to yr query although oriented to BRC Standard. In particular see the document linked in Post 5 which explains the Certificate data quite well IMO.