pitot,
i believe your 737NG's engines are flat-rated at ISA+15°C. this means the engine will produce it's max power at ISA+15°C. At any higher or lower temp, the eng will produce less thrust (thats why the lowest assumed temp is ISA+15°C).
say you are taking off at OAT 30°C. the thrust produced by the eng will be higher than if say the OAT is 45°C, right? by entering an assumed temp of 45° in the FMC, you are merely 'fooling' the FMC to think that the OAT is 45° instead of 30° thereby producing less thrust.
if you enter an assumed temp of less than ISA+15° (30°C), the thrust will remain at the current OAT setting. therefore, you will not get any thrust reduction as the FMC is not program as such. (try it in the acft!)
SR