The standard procedure before surrender was to disable your weapon. And the simplest way to disable a rifle is to take out the bolt and throw it as far as you can. Of course this only accounts for rifles found with non-matching bolts. But for total miss-matches there was often another reason.
When battlefields were cleared up, all the weapons were gathered and sent back for repair. Good weapons being made up from damaged ones and then re-issued. The military were never too fussy about matching numbers as long as the gun functioned properly.
The standard procedure for rendering a Vickers gun useless before surrender was to remove the lock, throw it away, rest a grenade on top of the receiver side plates and remove the pin. Job done!