This should be entirely unsurprising, right?
For the active force, you have to figure most of the people with around 12 years of service or more have decided to stick it out to 20 years and get the retirement. They'll think long and hard before turning down the shot and losing all the time they've invested.
Reserves/Guard, especially the part-time folks, don't have near the same incentive. I don't know all the nuances, but if you're strictly a part-timer, don't you have to wait until you're 60 to draw any kind of retirement? So many folks in the Guard/Reserve probably just said "thanks, but not thanks."