One presumes they picked the most serviceable and reliable they had...
....VMFA-121 also left a seventh jet behind at its base in Arizona due to a malfunctioning integrated power package (IPP), which provides electrical power for the aircraft, Bardo noted. In all, fewer than two of the squadron’s fighters— 23 percent—were “full mission capable” at any one time, on average. Crews kept approximately 53 percent of the six planes “partial mission capable” throughout Red Flag 16-3. “Notably, these numbers only reflect the six aircraft deployed to Nellis AFB,” Bardo stressed.....