At 40k, divided by 20 year work-to-retirement year should equal 2000 per year retiring, or 166.6666 a month.
I'd bet a dollar that certain months are far more popular tha others... whatever month or months their academy starts classes (assuming they only have a few classes a year, and not 12+).
Still, I'd also bet that their personel/retirement service admin types have a very good handle on the average # per month- probably for every month of the year, and I'd think if the average was really ~167, then an extra 60+ that month would definitely be noticable...