The chart page doesn't explain things very well, so it takes a bit to unpack, but the point of the chart is actually to give a better idea of comparative performance over a time period rather than focusing on a particular number like average return. Basically it's dividing the current value of one portfolio by the other at each point. The ratio shows the ebb and flow of the two portfolios against each other. John Bogle's speech [1] and this forum [2] probably explain it better.
[1] http://www.vanguard.com/bogle_site/sp20020626.html
[2] https://www.bogleheads.org/forum/viewtopic.php?t=138973
EDIT: cleaned up the first paragraph.