A mate of mine told me a story about him being pulled over doing 125km/h in a 60 zone racing a bike.
The female officer in the unmarked car said she was going to send him a ticket for something like 20 over the limit but it never arrived.
I think this is the same case. It's a cops job to book you for speeding, but if he says he'll do it later and it "slips" his mind, then he can't get busted for it and he's let you off.
Cheers,
Tim