There seems to be a lot of mis-information in this thread. Left Hand Downwind means when you are flying downwind, the runway is on your Left. Giving faster joining traffic a different circuit pattern is very common. For instance I am flying into Oxford in a Falcon 2000. There are several PA28s on a Right hand circuit on R/W 19. If I join the circuit on a Right Hand downwind I am going to overhaul the PA 28s no matter how slow I fly keeping at a safe speed for my aeroplane.. So I am instructed to join downwind Left so I do not conflict with them.
It is not a mistake by ATC nor is it confusing. You are being asked to fly a different circuit pattern for separation.