Quote:
Originally Posted by Bdot
Can someone please explain why P1 was able to find this factor?

Check your results.txt file. If you see the parameter E=6 (or some higher number) in the status line, then the BrentSuyama extension to stage 2 was used, which will occasionally find a factor with one prime factor of k > B2 and all others < B1. This optimisation requires additional memory, so it is not usually in effect unless the available memory is particularly generous.
There are other possibilities, even without the the BS extension. The Stage 2 code uses an optimisation called prime pairing, which sometimes includes two primes between B1 and B2 per iteration, instead of the one per iteration that a naive implementation would do. This is primarily a speedup  you need fewer iterations to include all the primes  but it does have a potentially beneficial sideeffect: Stage 2 can find a factor whose k has two prime factors between B1 and B2 if those primes happen to be paired. The likelihood is small, and I have never heard of such a case.
With the BS extension there are yet further possibilities, such as Stage 2 finding a factor whose k has one prime factor between B1 and B2 and one > B2, or more than two > B1. The likelihood is even smaller, and again I have never heard of such a discovery.