Rimfire ammunition comes in a bunch of flavours, including loud, louder, quiet, more quiet, and quietest. Aguila, for example, sells both 'Colibri' and 'Super Colibri'. Apparently, the 'Colibri', which is quieter, is intended only for use in short (ie.: handgun-length) barrels, and the 'Super Colibri', which is faster than 'Colibri', is intended for use in longer (i.e.: rifle-length) barrels.
It's not difficult for me to imagine that used surplus rifles which might have been used by multiple users over multiple locations might have been used with odd ammunition (which might or might not have ensured that the bullet left the bore), and it's not difficult for me to imagine some of these same users sometimes having been unaware of that fact (one way or the other).