Human perceived brightness is something like a logarithmic function of the power used to make the light. When you see most of the watch face go almost dark and

Human perceived brightness is something like a logarithmic function of the power used to make the light. When you see most of the watch face go almost dark and the important bits being "half brightness", that could be operating at <5% of the normal power. The new fancy display only needs to be refreshed once per second which lets all the switching electronics go to sleep. What that all adds up to is the always on time taking very little power and turning it off will not gain you much run time.