Can you do a measurement series with different temperatures (ice water, boiling water, etc.) also writing out the raw read value (RTD_CH0.rtd_res_raw) and posting the results?
What I think is very interesting:
tmp = ((double)RTD_CH0.rtd_res_raw / 16) - 256;
The resolution of that calculation has always a resolution of 0.06. That means the wrong result was when you got a 0.03 resolution.