vjs,
The setting is not based on whther all those numebrs have already been factored but on the calculation the P-1 code makes on the probability of finding a factor given that the number has been sieved to 2^48. Clearly, if you tell sbfactor that the sieving has happened till 2^42, the probability of finding a factor it will calculate, will be larger than at 2^48 and as a result it may think it is worthwhile to do a P-1 factoring. But in reality it will not be worth it as the factor probability is based on incorrect information.