It is not commonplace to use a gross including an already negotiated rebate.
It's like selling an item and claiming you bought it for the "list price"; when you paid less.
The net part is debatable, but including the rebate in the gross means that it was a
Because the uk simply didn't send that money (the debatable part is that large parts of it was returned); as only the money after the rebate was actually sent to the EU (and then partially returned).
Same as when you lied about the uk having more paid vacation than in the EU, when the uk has the EU-minimum of 28 days; it's just that the uk employers tried to cheat by counting 5-day weeks - that's why the uk has more "weeks", but the minimum amount of days.
I doubt that number is true as well.