Red5, what are you using to view the video and take screenshots? Your viewer is not interpolating/upsampling the chroma when converting YV12 into RGB - you can tell by the blockiness at the extremities of the strong reds.
I seem to recall you get this if you load an AviSynth script which delivers YV12 into VirtualDub - you can get smoother results if you add ConvertToRGB() at the end of the script.