There is no magic, everything is rational according to the theorem of Bernard ROSSET :
All pulleys, wheels, gears composed of two arcs of circles rotating around the axis of their common chords, with a constant speed to their girth, have a speed that varies periodically along the length of rays used.
The change in radius is 21.4% more and 42.8% less in total which is quite related to the given force to the cranks.
Rotations of the OGIVAL
Demonstration of chainring movement