If contour lines intersect the graph borders, the space between the contours is not properly (smoothly) filled.
Also, some of the closed contours in the interior of the domain are not smoothly filled.
See attached example.
The original example is confusing: it mixes a colour density plot (the most accurate approach) with a quantized color fill.
The problem here is simply that quantized color fills have not yet been fully implemented, as illustrated in the updated example.