In Fireworks, when you export a PNG8 file with alpha transparency, the resulting file will be something like this:
png8-fireworks.png: PNG image data, 500 x 500, 8-bit colormap, non-interlaced
If you convert a 32bit PNG using other tools (PNGOUT, Smush.it) the result looks like this:
png24-smushit.png: PNG image data, 500 x 500, 8-bit/color RGBA, non-interlaced
png8-pngout.png: PNG image data, 500 x 500, 8-bit/color RGBA, non-interlaced
What exactly is the difference? They both have alpha transparency, but the Fireworks file is 8KB while the others are 20KB. Now the Fireworks file in noticeably lower quality (namely with banding on gradients).
For some images the PNG8+alpha from Fireworks works great and has a super small file size comparatively. I just haven't been able to figure out what Fireworks is doing and how it is different than the other methods.