The answer may simply be stability. ANV operations generally covered the same geographical area, unlike the Army of Tennessee and the earlier Army of the Mississippi. Muster rolls often are carried forward through February of 1865, with corresponding medical records, receipts and requisitions. Most of us have a fair idea of how much of our personal things are lost, destroyed or tossed out during a move. The same must have been true for the AoT as it moved from place to place, often under duress.
ANV units serving with Early in the Shenandoah Valley in 1864 lost many records as a result of defeats at Winchester, Fisher's Hill and Cedar Creek. Records of these units are similiar in quality and quantity to those of the Army of Tennessee.