Correlation between 1H and 13C shifts - coincidence or not?
Maybe it would help to comprehend the coincidental changes in 1H and 13C chemical shift by understanding the origin of chemical shift. In both cases, very crudely speaking, the influence of ring currents and electronegative elements/groups cause a downfield (away from zero) shift. The factor of 20 quoted is a function of the number of electrons (usually outer ones) involved in forming the bond. So the trend will also be true for other elements e.g. 15N where aromatic N compounds i.e. pyridine have a larger downfield shift.