Why is the temperature accuracy different between end-products such as FLIR One, AX8, Lepton Radiometric?

Lepton cameras do not come in different accuracy grades. The difference between the various Lepton based products is in the system design and system level calibration.

In a typical system, Lepton cameras will typically achieve temperature measurement accuracies as called out in the Lepton datasheet, i.e., +/- 5 degrees. In order to achieve higher accuracies, contributions to accuracy from the final system must be considered. The design must not subject the Lepton to varying thermal effects. Also, calibration parameters can be created for the Lepton as installed in the final system. This can either be done based on a sample of these systems, or, for the best accuracy, calibration can be performed on individual units. This allows compensation for the window used, and thermal effects from nearby electronics. Details are provided in the Lepton with Radiometry datasheets.