I've been doing some testing for BC in my basement these past couple days and decided to finally finish sighting in my Summit by running some JSB Express through her instead of what I've always considered "break-in" pellets, CPHP. Just for kicks I threw a few over the chrony and was pretty confused that a pellet the same weight would be 30-40 fps slower so back to the CPHP and still holding true in the 830s. Back to the JSB and still down to 790-800 fps, including a huge amount of dieseling not present with the Crosmans that I've washed. Weird because I thought the slightly looser head fit of the JSB would help velocity compared to the tighter fitting pellet but no complaints about the rifle shooting cheapie pellets quick and dead on at 30 yards.
Anyway, lesson learned on trying to rely too heavily on mathematics instead of hands-on experience.
BTW the .22 Polymags had a MV of 771ish fps (estimated average) and came out with a BC of .024
To go along with your saga, I have also seen significant differences between boxed cps ( which were consistent on their own) and several other brands (not explained by the weight difference).
I even have 2 rifles in particular that shoot AA and JSB at notably different speeds over a chrony (with very different POI so they are not interchange at att.