Terms:
Production: UPT and FTU output. UPT is producing near 100% capacity, as are the FTUs of most MWS's.
Absorption: the capacity of an MWS to take in UPT grads and make "experienced" aviators. Limited by iron, flying hours, UTE rates, and unit manning, among many other variables.
Experienced pilots: Ones that have met the experiencing criteria of their weapon system. They can then fill staffs, ALO billets, teach UPT, teach at FTUs, etc. They also fill other flying billets ("non-absorbable") that require experienced pilots (WIC IP, aggressor, some VIP/SAM MWS's, E-4, test, etc).
Attrition: loses from all causes
RL/BL - the Red Line/Blue Line charts produced by AF/A1PP and A3O-AT. RL is the requirements (billets), BL is the actual bodies (manning).
Right now the AF is producing at near max capacity, and it's just enough to keep the total number of pilots at 100% of requirements. So, for the foreseeable future the Total AF Pilot RL/BL charts are matched up. But it's the mix of pilots within those RL/BL totals that's the problem.
The AF drastically cut fighter production (BRAC'd 2 FTU sq's at Luke, etc) and absorption capacity (fighter redux, TAMI21, etc) several years ago. But they didn't cut requirements for experienced pilots (UPT IPs, ALOs, staffs, aggressors, WIC, etc). So the fighter BL is dropping fast. The FTUs can't push them through fast enough, and then there's not enough iron and flying hours available to experience them.
On the other hand, mobility production hasn't decreased, and they've been using OCO flying hours to absorb and experience their pilots faster then they would be able to normally. You can send a UPT grad to a C-17, and have him come out as an experienced pilot in half the time it takes to make an experienced fighter pilot. Then you can use said experienced mobility pilot to teach at UPT, or work on a staff, etc.
When you break the RL/BL charts out by community, fighters are and will be undermanned, but mobility is and will be overmanned. Mobility overmanning was directed by CSAF and is intentional in order to keep total AF pilot manning at 100%. The AF can't afford to stop the overmanning of mobility. If we did, we wouldn't be able to fill all the pilot requirements. Mobility will continue to pay a higher share of the bills - UPT IPs, staffs, etc - than they would in a perfectly balanced world.
So when a mobility recallee asks for an extension, it's hard to justify approving it given mobility manning, total pilot manning, and the AF's total manning issues combined.