Speex dev - Dec 2005 - a question in jitter.c

hi,all
  In jitter.c, the function speex_jitter_init has following codes.

   speex_decoder_ctl(decoder, SPEEX_GET_FRAME_SIZE, &jitter->frame_size);
   jitter->frame_time = jitter->frame_size;

  Why is frame_time is equal to frame_size. As we know,if we use 8k
sampling, the frame_size is 160,
(frame_size = 320 ,sampling = 16k ..),but the frame_time is const, it is
20ms.

Best regards
keith