It is actually quite easy to do that. If back in 06/2010 you had 2 lines, and one of which was a 3GS that was not eligible for upgrade yet, and the other had another phone which was eligible for upgrade, you could have used the eligible upgrade to buy an iPhone 4, then activate it on the other account. Now, today, the account which has the iPhone 4 is past it's "2 year" mark, and is now eligible for an upgrade.
There are many discussions in these fourms about how to use the upgrade form another account to get an early upgrade" on another account. This easily explains what you are seeing.
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