Whay think do FMC OPT ALT Change in when Selecting Exactling the same TGT max

FL 360 -PIF Razzaki
When Using ECON mode fmc calculate optimum alt using Cost Index (speed is caculated to get less fuel or less time ) and when selecting same mach number it changes from ECON mode to speed mode and calculate the opt alt for minimum fuel consumption using the selected speed.😎

same