Most print runs are slightly more than the stated number -- for example, if you are going to print and bind 1000 copies of a book, the printer/bindery normally does a few more, perhaps 1100 or so, to account for any copies that are damaged in the printing process. So if 1000 copies are desired, 1100 are printed, and 40 are damaged during production, the company still has 1060 copies. I suspect all the unnumbered copies of the SR3 hardback that are floating around are from this "overage."

I know FASA sold some of the unnumbered ones at GenCon one year, and I suspect they also went out as complimentary copies to friends, but it's possible that a bunch of them stayed "hidden" at the FASA warehouse until they closed, and then someone managed to snatch and sell them. A lot of publishers maintain a "history" of the company, keeping a box [6-24, generally] of sealed copies of each product in some dusty corner of the warehouse.