I see this type of question a lot and, conceptually, the thing to understand is that you are bouncing an _OUTPUT_, not a track. Even though tracks are what we are working with and are the content that are involved in the bounce, technically speaking, it is the output we are bouncing.
So, if you bounce a stereo output, you are going to get a stereo file. If you bounce a mono output you get a mono file. Logic, by default, sets things up with everything routed to a stereo output, so it is sort of presented to us in this way right from the beginning.
As Pete says, you need to create a mono output first. And yeah, panning (or panning related effects processing) would be the only explanation I can think of too that would explain why it is coming more out of one side than the other.