Hi,
In swscale.c, there are some lines that adjust {lum,chr}XInc depending
on MMX or MMX2 being used for fast bilinear. They were added (still
with the old name of s_xinc) in this commit:
http://git.ffmpeg.org/?p=libswscale;a=commitdiff;h=87d2542e33c80731dab64339aebcaebbaa25911e
Where do the magical "20" values come from?