Here is some info I found on Wikipedia:
The 9×21mm pistol cartridge (also known as the 9×21mm IMI or 9 mm IMI) was designed by Israel Military Industries for those markets where military service cartridges, like the 9×19mm Parabellum, are banned by law for civilian use, like in Italy.
And this I believe is very pertinent to this discussion:
Based on the 9×19mm Parabellum cartridge, the case was lengthened from 19.05 to 21.15 mm (0.750 to 0.833 in). The bullet sits slightly deeper in the case, which results in almost the same overall length as the 9×19mm Parabellum cartridge (29.69 to 29.75 mm (1.169 to 1.171 in)).
Here is the picture they include in the article:
Since the overall size of the 9x21 and the 9x19 is approximately the same, I wonder if it is all that surprising that a 9x21 will fit in the 929 cylinder.
Here is the citation of the Wiki article I quoted:
9×21mm - Wikipedia, the free encyclopedia