To add to the previous good answer some intuition as follows with optics and waveform required.
In optics we have corrective lens that depend on the spherical and cylindrical correction opposite to the eye’s deficiency. The spherical diopters either magnify or condense according to the limitations of near or far field focus respectively. Astigmatism is the cylindrical lens corrections which create the elliptical distortion of pinpoints are corrected by the linear cylindrical curve across the horizontal axis.
Similarly, magnetic stigmatic correction is done by a sawtooth weak current to stretch or contract the horizontal sweep of the electron beam. The diagonal opposite gets the inverse current so that axis has a null sum of stigmatic correction to create a slight cylindrical correction for each axis to result in a spherical beam. The transition ramp occurs during the blanking horizontal retrace interval.
With 3 of these, astigmatic correction can be made for each RGB colour which is emitted from nearby yet different parts of the gun.