i have been using deflopt for along time . i use it after optipng then advdef or advpng the result is not just 20byte.. actually it varies based on the images some save byte but others save 10kb .. specially on texture images used for rendering.. i could figue it out.. how it can act that will on some files but not others..

on ther hand pngout is best but veryslow there is no quick setting to reach the smallest file..

anther program you can test is http://psydk.org/PngOptimizer
but i noticed that in some palette images it can invert color or introduce colors that does not exist