Because it was the easiest solution
and I wanted to actually get a YV12 version out to see how it performs with people using it on proper YV12 sources (not just me testing it with a ConvertToYV12 before it.)
AFAICT there are a number of issues such as was the source treated as interlaced or progressive when it was MPEGed and therefore does the 1st U/V plane line contain info about Y plane lines 1 or 2 or 1 and 3 etc
So my brain was hurting and it looked OK on my setup so I decided to publish and be dammed (which you've so kindly done
regards
Simon