24 or 28 bit samples?

Hi all. Possibly a dumb question...
MAD_F_FRACBITS is set to 28 by in mad.h; does that mean there are
28bits per sample in mad_synth.pcm.samples?

I'm applying dither during the conversion to 16bit samples, and if I
treat the samples as 24 bit I get terrible output, but if I treat them
as 28 bit the audio sounds fine (with limited testing though).