If the available fault current is 42K, then you need to go to the next higher AIC value. The IEEE short circuit calculation method allows some simplifcations, and the results should always be used with a margin of error.
Although, not foolproof, a reasonable guess at the AIC rating of the gear, is to find the smallest rated OCPD, and use that value. If the main is 100kAIC, then it is fair to assume the board is too. If the board was not required to have a 100kAIC rating, than surely the gear manufacturer would have provided a smaller AIC rated main to save money. This method is not foolproof, but it is a fair assumption.
In addition, the engineer of record might have wanted to plan for future distribution modifications. Maybe parralled generators, redundant utility services with closed transition, etc.
