Looks like color management trouble, even though the difference is pixel noise. http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=compositing%2Fimages%2Fpositioned-image-content-rect.html
Marked as flaky in <http://trac.webkit.org/r156012>. The test is not really flaky, but that's the best we can do given that both WK1 and WK2 bots are failing, but not all of them are.
We now enforce sRGB, and the test passes on all bots. I'll unmark it.