Hmm.
I came accross a few possible "reasons" for this (via BBSH lectures & personal discussions)... One that struck a chord is that all humans (& all souled entities - essentially everything!) is part of an "oversoul." (Other ideas being that the soul/re-incarnation does not happen in a linear time path - be reincarnated at multiple times in the same span, future/past, etc...)
So... You have a person with their own spirt... Which is part of a larger divine structure. And... That structure can have more than one enpersonified manifestation.
Some "proof" of this was how people under past-life hypnosis had the same past lives... (Can be taken as evidence depending on what you think of past-life hypnosis experiences.)
So... A "soul mate" could essentially be another piece of you.
Thoughts?