Created attachment 207918 [details]
(This shares the same preconditions as bug #74131)
If you have an element in a region with a percentage margin, and the region is followed by another region with a larger width, the size and margin calculations are using the widest region in the flow, then moving and clipping for display in the first region. As a result, using getComputedStyle to retrieve these margins fails, giving the original values which used the widest region in the flow.
I created a test, starting from the one attached in bug #74131 and modifying it to incorporate the logic from http://trac.webkit.org/browser/trunk/LayoutTests/fast/css/getComputedStyle/getComputedStyle-margin-percentage.html. It can be seen how the test passes once we remove the second region with the larger width.