I think this firearm in .40 caliber comes standard with 12 round mags. A 10 round magazine would kind of make sense, but the 12 round wouldn't make a huge difference if buying ammo in a 20 round box, you'd maybe load each mag with 10 rounds....assuming a loaded magazine is inserted in to the firearm, then the slide is racked, that would leave one round in the chamber of the gun, and 9 left in the magazine.
My guess/speculation and it's totally just that, if BG had 9 in the magazine, and one in the chamber, and racked the slide at the CS, then that would result in of course, that one bullet found on the ground at the CS, and the 8 rounds left in the magazine, BECAUSE
when you rack the slide, one bullet ejects, and then another is chambered. So, 10 rounds, rack slide, 9 left in mag. and one in chamber, ready to go, then at CS, rack slide, one round pops out on to the ground, another chambers in the gun, leaving 8 in the magazine.
Keep in mind, this is all conjecture, and may easily not be fact in this case, but kinda makes sense, to me anyway.
The ammo could have been purchased in different box sizes, but a hunch is, being a self defense ammo, it was purchased in a 20 round box, which would make sense too, as two ten round magazines would be loaded from that one box, (or 10 rounds loaded in to a 12 round magazine) which jives with what is listed as being found on the aforementioned document.