Alta- Your question got me intrested, so I checked his compiled service record at the library on the way home. I found it intresting that the muster rolls indicate he was mustered out(discharged from the army) for disability on Nov. 30, 1862, a year before his capture. Also I suspect the Tenn. river entry is a mistake. Since at his capture he still claimed to be a member of the 8th, (which was several hundred miles away in Georgia), although discharged a year earlier, the most probably scenario I can come up with is he was left behind in Kentucky with injury or illness during the retreat from Kentucky following the battle of Perryville in Oct, 1862. While his unit was at Murfressboro it was determined he would not be fit enough to rejoin the unit and he was discharged. Eventually he was picked up by federal forces, and possibly not knowing about the discharge, gave his unit as the 8th TN. To be caught that far behind union lines and not be treated as a spy he must have had a good story to tell the Feds to explain his presence. Note this is just a theory, there's probably no way to know the truth for sure. It's also possible he was running with a band of Partisans and used his former status as a soldier to avoid be treated as an outlaw and just forgot to mention he was no longer actually in the army.
It's also intresting he decided to take the oath of allegiance to the Union and was released in March of 1865, before Lee's surrender in April.