at least current libvpx code makes it's own decision haw many threads to allocate. The threads options provided by ABI is just maximal restriction.
At least for me it makes difference.

IMHO if we can force count of threads, it should be called "force-threads",
if we can only suggest max count of threads, then it should be called "max-threads".

If you have other suggestions for names, or other ideas. you are welcome.
The part of this bug is also here:
https://bugzilla.gnome.org/show_bug.cgi?id=673891

here are snips of code form libvpx, responsible for threading decision.
libvpx/vp8/encoder/ethreading.c:
void vp8cx_create_encoder_threads(VP8_COMP *cpi)
...
  /* don't allocate more threads than cores available */
   if (cpi->oxcf.multi_threaded > cm->processor_core_count)
       th_count = cm->processor_core_count - 1;

  /* we have th_count + 1 (main) threads processing one row each */
  /* no point to have more threads than the sync range allows */
    if(th_count > ((cm->mb_cols / cpi->mt_sync_range) - 1))
     {
        th_count = (cm->mb_cols / cpi->mt_sync_range) - 1;
     }


libvpx/vp8/decoder/threading.c:
void vp8_decoder_create_threads(VP8D_COMP *pbi)
  /* limit decoding threads to the max number of token partitions */
   core_count = (pbi->max_threads > 8) ? 8 : pbi->max_threads;

  /* limit decoding threads to the available cores */
  if (core_count > pbi->common.processor_core_count)
     core_count = pbi->common.processor_core_count;