There's always the chance that some, or even many, of the captured wounded were taken into Chattanooga. I had an ancestor that was in a hospital there a year earlier before the Union occupation. From what I understand from a park ranger many soldiers were buried in close proximity to the Tennessee River (mine was possibly one of these) and a flood in the late 1860's washed away the markers and even uncovered some of the bodies. The graves were moved to higher ground in what is now the Confederate cemetery, which is why there are so many unmarked graves there. Also, a lot of the marked ones are probably not over the correct body since they couldn't identify them after the flood with no markers, they only placed new markers from what records they still had on file.