It sounds like to me since these are O.R. and Congressional "official figures" that they are not talking about anything that had to do with researching the notations on the Muster Rolls records of each individual soldier of all the units. This sounds like to me that these are the figures of Confederates who simply walked across 'No mans land' and entered the Union lines and gave themselves up.

Now the Union figures of deserters would have been something else.