There were a couple of factors at play here. First, the federal government offered incentives for the veteran (experienced) soldiers to stay in the field and extend their enlistments. As mentioned above, it was $400 and a 30 day furlough. But keep in mind that the states, counties and municipalities also had draft quotas to meet. Some offered enlistment incentives of their own, such as a $300 bounty from the state, $100 from the county, and $50 from the city if one voluntarily enlisted and reduced the number of men who needed to be drafted. The federal government allowed reenlisted veterans to be credited to local draft quotas, and some of the localities paid their enlistment bonuses to the veterans as well if they reenlisted and were credited to their quota. Look through the records for any unit with a large number of reenlistments and you will find several disputes about where reenlisted soldiers were credited.