For multiple pass welds the original hardness gets tempered so the hardness is reduced. The current model predicts a single pass weld so it is a worse case scenario since no tempering has occurred. The conservatism of the model prediction compared to a multipass weld is what would be expected so as long as the PRCI weld hardness is acceptable.
This creates a high level of confidence that a multipass weld hardness will be less resulting and in-turn lowers probability of cracking. The current model is not capable of modelling tempered weld hardness.