If I had to guess, folks at the ATF were tasked with finding some examples. While the BATFE/NICS doesn't know how many or what make/model of firearms someone buys you'll never convince me they can't pull up how many transfers a name/dob/ssn have associated with them. Those transfers will have FFL numbers attached indicating a pattern of transfers from the same numbers. Once someone decides to look the data is there.
Good point. It probably wouldn't take but a couple milliseconds to run a report by buyer to figure out where to begin looking.