"I have to think that most of these "desertions" are reserve soldiers, sailors, marines, airmen, etc. not showing up or refusing to oblige their contracts for active duty."
Some of these desertions are Soldiers, Sailors, Airmen, and Marines who are facing Court Martial or NJP for other offences, and they don't want to face the music. My unit had two of these in Jan. They were being prosecuted for fraud, and never returned from the New Years 96.
During my career, I had only one man dropped from the roles as a deserter. He decided he needed an extended vacation, but forgot that it included a 31 day month. He reported back late on what he thought was the 30th day – it was the 31st day.
I had the paperwork and MPs waiting as he strolled into my office. He stayed in the area and I had been keeping track of him.
I would have thought much the same way. One item I read claimed "desertion" did not apply to the Guard and Reserves. Continuing my search, I found this which is more recent and appears to contradict the above info.
https://www.nm.ngb.army.mil/DCSLOG/Army%20Regulations/r630-10.pdf
Desertion Rates
Fiscal Year - Number of Deserters - Rate Per 1000
Army:
1997- 2,218 - 4.58
1998 - 2,520 - 5.20
1999 - 2,966 - 6.13
2000 - 3,949 - 8.16
2001 - 4,597 - 9.50
2002 - 4,483 - 9.26
2003 - 3,678 - 7.60
2004 - 2,376 - 4.91
Air Force:
1997 - 26 - 0.07
1998 - 27 - 0.07
1999 - 45 - 0.12
2000 - 46 - 0.12
2001 - 62 - 0.17
2002 - 88 - 0.24
2003 - 56 - 0.15
2004 - 50 - 0.14
Navy:
1997 - 1,858 - 4.86
1998 - 2,038 - 5.33
1999 - 2,485 - 6.50
2000 - 3,255 - 8.51
2001 - 1,619 - 4.23
2003 - Not Available
2004 - Not Available
Marine Corps:
1997 - 1,375 - 7.94
1998 - 1,460 - 8.43
1999 - 1,689 - 9.75
2000 - 2,019 - 11.66
2001 - 1,310 - 7.57
2002 - 1,136 - 6.56
2003 - 1,236 - 7.14
2004 - 1,297 - 7.49
BTW, I found this interesting tidbit while searching for answers: The groundbreaking ceremony [of the Pentagon] took place on September 11, 1941.