In the above example, the A1 temperature was calculated based on a nominal composition of P91. This temperature can vary with composition and can be different between the weld metal and base metal, but this is hard to capture for every possible heat of material without performing many experiments. With Thermo-Calc, you can calculate this across the entire composition space.
The histogram here shows the A1 temperature calculated for 200 compositions each that fall within the P91 base metal and two weld filler metal matching composition specifications. As can be seen, if the post weld heat treat temperature is selected according to the base metal compositions, the A1 temperature could be exceeded in the weld metal resulting in fresh martensite formation.