STRANGE artefacts with mp4 export

HI all.

I have noticed that when I use compressor for mp4 in a html5 web page, the very bottom of the video shows strange banding. The video played locally with quicktime shows only a very subtle artefacts in that area which are actually very hard to see unless you are looking for them.

The strange thing is, the same video compressed using Prism (free version) is fine. Also, using Compressor to export .mov is also fine.

it's very very puzzling.

Any ideas would be much appreciated.

The links show the same clip with is only about 20 seconds long

compressor mp4

prism mp4

compressor mov compressor video sharing preset