Why are some people born with one kidney, and others two?

Good question: Basically we often don't know. Every embryonic fetus has potential to develop 2 kidneys. All mammals have fetal metanephric nephric cells which divide thousands of times 2 develop into kidneys. Additionally iny "buds" develop on either side of developing fetal bladder, these buds lengthen & fuse with metanephric cells exactly in their center. Failure of any stage can result in no kidney that side.