"It's because if you lose Left and Center hydraulic systems, you lose Stab Trim (although u can get it from the right thru the PTU). In the B757, if you lose C & R you lose all autopilots."
I am not sure what you understand at the moment, but my manual is quite clear. With the loss of the Left and Ctr Hyd systems the both stab trim systems are lost. A Pitch Enhancement System (PES) uses trpped left trim system fluid to operate the stabiliser. Hence it is possible to use the electric trim at about 1/4 rate, automatic and alternate trim are not available. Thus, the autopilot will not, technically, be available. Testing it on the ground may allow it to work as the elevator can probably put in any demand, only when the stab trim tries to operate will the autopilot decide it is not going to do it and trip the autopilot out. It is actually possible to use the right autopilot as a sort of attitude hold with the left and ctr hyd systems failed, but it will trip out when stab trim is needed (ie any major attitude change).
Anyway, thats how I understand it!