You'd have to ask the person who placed the marker. Evidently that person didn't know that the 115th Alabama never existed. Perhaps the same person misread a handwritten number 4 and thought it was 11. The number 5 is correct.
The original post about this man seemed positive about James E. Warren's dates and military command.
Records of the 45th Alabama Regiment are limited. An unfiled record in J. E. Warren's name shows leave of absence granted on March 4, 1864. A pension application to the State of Alabama from Geneva County was made on May 13, 1893. When did this man remove to Thomas County GA?