Some interesting analysis here, but I think you are analysing a video far too heavily in the spatial space i.e at frame level, this doesn't particularly give reliable results as compression and hence artifacts can vary according to frame in time.

Video needs to be analysed ideally in the temporal space (i.e as a sequence). I see no mention of the GOP structure or length of the encoding chosen, which would need to be considered.

For example the one frame you have chosen to compare could be an I frame in some of the video compression or could be a P or B frame which would result in slight variance in quality and artifacts.