Gysbreght: As others have pointed out - although the STATIC thrust in the flat rated regime may be constant, it changes by a variable amount with temperature during the takeoff roll. Hence thrust variation. Add to that, ref. to the Boeing paper (Vilas posting) shows also that there is a thrust difference at FLEX with respect to the ambient temperature.
Thus the consensus seems to be that, yes there is a variation in gradient!
Which I think is what you said.