This appears to be what Heathrow has been doing for a little while, taking the boarding card again as you approach the security point to measure the queueing time, which is reported in government-required statistics.
I've written before how an ensuing fraud then started at Heathrow (certainly T5) with the operative taking the cards at the security point works progressively back down the queue, taking the cards, scanning them, and giving them back to you increasingly early, when there are still maybe half a dozen people (and maybe some minutes) to go before you can even start putting things on the belt. Furthermore, to get people up to the front of the main queue even earlier, there now is normally a second sub-queue of half-a-dozen or so people waiting for their turn through the scanner.
My hunch is that the "someone jumped the first security point" line is a parrot answer they have been told to give if the precedure is questioned. because otherwise people would start saying that their cards had been timestamped again long before they get to the head of the queue.
The only way to measure this properly is to scan again at the final exit after all the procedures have been completed. That would be the genuine security time. Of course, the officials at the DfT who thought this requiement up would have no clue about how to implement it or give scope for fiddling it.