It may be difficult for the rider to determine power alone. Going into a turn is more like a short free fall. In certain circumstances it can be difficult to turn the legs fast enough to hold the power from the straight. Going out of a turn one has to lift his weight, thus one often sees a power spike here.
As faster the cyclist, as closer he stays on the black line, as smaller the curve radius and as higher the mass the more difficult it becomes to avoid power fluctuations. Going over the black line when turning into a curve helps to even out such power fluctuations but then one goes a longer distance.
Attached some track data from riders having different speeds, power, power distribution and mass.
