The data that is reported in Table 9 of the datasheet is based on a linear equation used to approximate the temperature based on the ADC code. The linear equation used is Temperature (°C) ≈ (ADC code/32) – 256. While the relationship between resistance and temperature for a Pt-RTD resistor is reasonably linear, there is some curvature that causes the inaccuracy called into question in Table 9. For the best accuracy, you will need to use the Callendar-Van Dusen equation (see page 10 of the datasheet). You can also use a lookup table to correct for nonlinearities. The MAX31865 will output an ADC code corresponding to the resistance of the Pt-RTD, and then you can use whatever method you would like to convert the resistance accurately to the appropriate temperature.