Every incarnation of Xehanort has tried to form Kingdom Hearts through incorrect means. Ansem SoD tried to create Kingdom Hearts using the hearts of the seven princesses. This however only created a Door to Darkness that was mistaken for Kingdom Hearts (honestly can chalk this up to not having the entire story thought out at this point yet.)

Xemnas tried to created Kingdom Hearts using the hearts of countless people. The plan that was told to Org XIII was that this would grant them all hearts of their own, but it was later revealed by Xigbar that the real plan was to use it to infuse all Org XIII members with Xehanort's heart. This is likely why he didn't make an attempt to forge the true Kingdom Hearts.

Master Xehanort in BBS was simply being impatient. He knew Kingdom Hearts was meant to be forged using 13 darknesses and seven lights, but he was overconfident and thought he could just use one heart of pure light and one of pure darkness. This clearly didn't work and after that he decided to do it the correct way, eventually using his heartless self to gather the seven lights and his nobody self to gather the thirteen darknesses.

And this is speculation on my end but it seems like those parts of him remembered what they needed to do but not exactly for what purpose, so they mistakenly thought each of their tasks would create Kingdom Hearts on their own, which clearly isn't the case.