Ulic,

As a dispatcher who has flight planned 757s, I can tell you that the MTOW diff you refer to is probably because of on engine type. Generally, the P&W engines were higher thrust rating than RR, therefore allowing a higher takeoff weight, although I haven't looked at the tables you have asked about. You may referring to maximum "calculated" takeoff versus maximum "structural". You are right about the FF/Eng per hour.

I have actual 757 takeoff perf charts I could email you.