Most likely some gamma chunk is to blame. You can strip that one with tools like pngout or tweakpng... once you did that you'll get straight rgb values.
Pngout takes its time, but the images are usually alot smaller after that.
Well, png is a nice format, but it isnt all that useful for games. With webstart you use compression anyways, which means that TGAs (or any other raw format) ends up at about the same size (but faster loading times) and if you're using an installer with lzma compression (eg NSIS) the download will be alot smaller with raw images, because the compression scheme is superior.