dvt
The 757/767 generally has Max thrust or standard thrust (slightly less). In either case, both these aircraft are generally takeoff with way too much power to get the job done.
The 757/767 that I fly have both fixed derates and assumed temperature derates and as a matter of course we always use the assumed temperature method unless something prohibits it's use.
Perhaps the 738 (which I am not familiar with) is operating closer to it's maximum performance most of the time?
PP