Why does 16 bpp look the same as 24 bpp?

[Rick Macdonald <Rick_Macdonald@shaw.wave.ca>]

I can't tell the difference between 16bpp and 24bpp 24 colour JPEG
files, viewed with xv.

I have an ATI Graphics Pro Turbo (Mach64). The 24bpp implementation is
actually packed in 32bits, so the xserver is actually called 32bpp.

Can anybody explain this? I would have thought that 65K colours would
_not_ look as good as 16M!

-- 
...RickM...