According to the datasheet for the XTR108, the registers with unused bits should be filled with '0'. Using your calibration software and development board it appears that for PGA Gain, PGA input and Iref, the software is filling with '1's. Can you tell me if there's any issues with filling unused bits with '1' instead of "0'.
Example of the register content after going through calibration using the development software for -50 to 150 PT100:
0 Control 1 = 0x00
1 Reserved = 0x00
2 Reserved = 0x00
3 Fault Status = 0x00
4 Control 2 = 0x01
5 Over/Under-Scale = 0x17
6 PGA Gain = 0xFB
7 Alarm Config. = 0x00
8 PGA Input Config. = 0x8A
9 I_REF Output Config = 0x8A
10 Fine I_REF Adjust = 0x47
11 Coarse I_REF Adjust = 0x0C
12 Fine Zero Adjust = 0x30
13 Coarse Zero Adjust = 0xFE
14 Linearization Adjust = 0xD6
15 Checksum = 0x7D