If you are doing the P1 factoring, you are not proving the number to be prime, you are rather looking for a factor to disprove its primeness. Because, you can't find a prime with P1, you don't get credit as having found it, therefore you don't share as a discoverer.
If you are doing the LL primality checking, you do get credit.
To answer blackguard's question about double checking: To ensure that a newly discovered prime is real, Geogre has some volunteers that have access to nonx86 computers that are fast to do double checking.
