Most likely using the dodge tool in post, depending on your monitor, he might not have intended for these to show and true enough he might not have been able to view these artifacts while working on it, but different monitors with different display properties (contrast/gamma/brightness... etc.) could show unintended artifacts that the creator/editor didn't see in their own setup.
If I'm not mistaken,
Lewd4D could be using GIMP (maybe even Photoshop), in that case you can change your image depth to 16bit or more if you want softer falloff with your brushes. Had the same problem once before, so yeah, hope thats helps