Has anyone any experience with a Confederate ancestor who has only a very small service record yet was said to have served longer ?

I'd like to piece together the service of my 3rd g grandfather of the 6th TN. Inf. Co. I. He has only a one page record dated Aug 12 1861 ( it mentions enlistment date of May 23 1861 at Jackson TN.); yet the 1890 Veterans census states he served until Dec. 1864 (No notes on discharge, etc...).

I'm not sure if I should believe the Dec. 1864 date or the one page service record which would indicate he served for a very short period of time.

Thank you,
D. Williams