Main reason is: deltas in interframes need scaling by 3/4 before applying.
Detailed description is at:
http://sourceforge.net/tracker/index.php?func=detail&aid=1222099&group_id=16082&atid=116082
He also mentioned some samples at:
http://www.nerdgrounds.com/indeo21_test/
patch posted to ffmpeg-devel by (Kostya: kostya shishkov, gmail com)

Originally committed as revision 4425 to svn://svn.ffmpeg.org/ffmpeg/trunk