it is an oddity.. the result of a mix of the BJ-3 being retroactively inserted into the setting well after the BJ-2 had been established in TRO3050, and the general FASA trend of not giving a [bleep].
honestly though i wouldn't be surprised if maybe the prototype for what would become the BJ-2 didn't exist already in setting (a BJ-2X or something), leading to the BJ-3.
given that it looks likely that both were built on St.ives. if the BJ-2 prototype had a lot of growing pains (or just a lot of redesigns) i could see a 20 year development cycle. especially if the design got shelved for a bit after the 4th succession war and then revisited around 3039. we've honestly seen worse in the setting.