My 2L6N number starts with "A" and was created by OnePass Customer Care in late 2005ish. I had an essentially abandoned pmUAMP account (used for one trip in 2003 or 2004 -- first 777 and 747 flights the never again) that got carried over with a 2L6N account (and then merged into my "A" account.

My best guess is that some number of accounts from the earliest days of OP (perhaps as far back as when it was shared with Eastern) were so dormant that they had been purged and made available for reuse. Now in the days of cheap storage and wanting a lifetime customer profile (even ignoring the "miles never expire (for now)" gambit) it's unlikely that any accounts currently in existence will be purged.

Even though the 2L6N created from the old UA account was merged if an agent knows where to look it still exists but if you try pulling up that customer profile in SHARES it auto-redirects to my original CO MP number, so I seemingly will be forever squatting on two of the "valuable" 2L6Ns

I do see very few of the 3L5N accounts in my causal observation and small company travel accounts management fwiw