Another possible reason for the high levels of wake are to do with the fact that the B757 inboard and outboard flaps are vitually one complete surface with no inboard aileron.
This gives a flap surface all the way from the fuselage to the aileron. This a good distace for a vortex to be generated. On a say a B767 a fair amount of this votex spills out from the inboard flap where is abutts on to the inboard high speed aileron.
SAM 2M
