nice pic! lol
As a mere ATCO I would like to join in.....
Maybe the pilot THOUGHT he was at 1500ft. (200 ft is only 7 mb or so)
It is always worth remembering that when we are verifying yr Mode C, we have a 200ft tolerance.
So - when you say level at 1500ft you could be showing 1700ft but you could also be showing 1300ft.
And before you say anything - am neither defending or flaming the pilot here!!!!!
Cheers