I absolutely don’t know where that problem comes from, but there seems to be a rendering issue under MacOS 10.5 Leopard with PNGs created using the GIMP. I stumbled upon this while I was working on a website design I created some time ago. I don’t know if this might be a feature of one of the involved tools but I don’t think so.
Since I switched GIMP and MacOS X versions simultaneously, I’ll mention the versions here to ensure the difference is reproducible. Although you may not see it (depending on your monitor), there is a subtle difference.