Alright, what's confusing about it? We don't know the specifics, but I think we're all well aware of the process by which one becomes a soul reaper. He was obviously in Soul Society and went through the academy, though we are still a bit lost on the time at which these things happened. Because of his power, he must have risen to prominence, and then, for some reason yet to be elucidated, he left Soul Society, went to the human world, had sex at least 3 times with a human woman and settled down with the family. It's not really confusing how he became a soul reaper since everyone follows the same general path to that. It's more confusing why he left.
It isn't confusing, I mean I have a feeling he and Urahara followed the same path. Being captains in Soul Society then for some reason, left their posts, returned to the real world in an attempt to live normal lives again. Or another theory of mine is that they could be guardians placed here by Soul Society so that they can keep watch on Hollow activity on the real world. Some sort of Shinigami Enforcer...
