That is because the GPUs he used are fairly low power at the expense of some performance. In the video he states he was initially planning for the Fury X but found that he'd need significantly more power to drive them. Also of note with regards to power supplies is that most are at their peak efficiency in 80% load or about that range. In that scenario you are buying a 1000w PSU for what you expect to have a nominal draw under full load of about 800W. You would be very hard pressed to hit that on a single card but for overclocked machines you can easily hit this especially if you use SLI.
There is that as well but your charts do suggest a variance in efficiency for higher level loads. This does depend on the power supply. For better power supplies it doesn't really matter, but do note that the drop on the high end is more significant to drops on the low end. (3% of close to 1000w is close to 30w, 3% from 200w is a 5th of that)
