This looks exactly like what should be expected. When you quantize your z data to 15 levels, you have less z-resolution than when you quantize your data to (e.g.) 256 levels.
x = linspace(0,1,50);
y = linspace(0,1,50).';
z = x+y;
imagesc(z)
colorbar
colormap(jet) % default is 256 in newer versions
figure
imagesc(z)
colorbar
colormap(jet(16)) % explicitly use fewer levels
It should be noted that when you don't specify the number of levels, the number of levels that's used depends on the version that you're running (defaults are either 64 or 256) and the last colormap that was used in the same figure (even if the figure has been explicitly cleared). If you care about the number of levels in the colormap, you must specify it explicitly.
% if the length of the colormap is unspecified, and we have an existing
% figure open, the length of the new colormap will be the same as the
% length of the last colormap used in that figure
clf % clear the figure
imagesc(z)
colorbar
colormap(parula) % default is ignored, and the length of the last map is used
% so explicitly specify the map length if you want a particular length
clf % clear the figure
imagesc(z)
colorbar
colormap(parula(256)) % explicitly specify the desired map length
This behavior is mentioned in the documentation for jet(), parula(), etc.
