This has been discussed before, but I could not find it
Anyway, all is conjecture, with several older members having witnessed #1 or heard it from stories, plus I have read it and seen a picture somewhere.
#1 - GI's when surrendering en-mass would take their holster off, and drop the pistol, magazines and holsters into separate piles or barrels.
#2a - As Ron said, in battle magazines were mixed up. In several luger books this is discussed, although I assume it is conjecture; that GI's just wanted mags that worked, not caring if they matched. Now, an armorer would match mags up when he had a chance and sometimes mark them or remark them / when he had time.
#2b - When taken off of police, they are more likely to be matched, I have seen more 'correct' police matched rigs than army
#3 - in the evening, cleaning guns or under fire but guns were dirty (trench warfare, WW1 esp) would not care if things got mixed up
#4 - US GI's probably could care less, as USA guns were not numbered like German ones were...
#5 - Items imported would not be matched and although they might have been matched at first in germany, once in a importers control, they just matched them up
Ed