Most scintillation effects are caused by anomalous refraction caused by small-scale fluctuations in air density usually related to temperature gradients.
Stars twinkle because they are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence which acts like lenses and prisms diverting the light's path.
Large astronomical objects closer to Earth, like the Moon and other planets, encompass many points in space and can be resolved as objects with observable diameters. With multiple observed points of light traversing the atmosphere, their light's deviations average out and the viewer perceives less variation in light coming from them. https://en.wikipedia.org/wiki/Twinkling
But I don't understand why light coming from one point is more scattered than light coming from more points. I would expect the opposite because the lines to the receiver are more close together. Besides that, how is the deviated light of a planets averaged out. Is the light (of the edges) of the disc (planet) really capable of getting a constructive interference or something like that? How could this work?