Its a bug, sometimes the Airgroups are "splittering" into different parts.
btw. you can also use:
(.. as AiAirGroup).InitNOfAirc; //start number
(.. as AiAirGroup).NOfAirc; //current number
(.. as AiAirGroup).DiedAircrafts; // number of destroyed
but these also work sometimes not correct at the moment, maybe the next patch fix the problem.
