[css-flexbox] percentage heights of flex item descendants

Hello,

so I saw that the flex box spec requires that a descendant of a flex item
with percentage height gets resolved against the flex item's actual height:
http://dev.w3.org/csswg/css-flexbox/#algo-stretch

I was wondering why that is? This is inconsistent with every other use of
percentages, and requires multi-pass layout (ie. is slow).

thanks,
-christian