When ol.Overlay has set positioning is set to right or bottom, no offset is applied. I added the missing calculations.
I made 2 fiddles showing the bahaviour before and after.
The divs in hte fiddles are named after their positioning (anchor point)! Not the actual position!
Before: http://jsfiddle.net/simon_seyock/ob7ggtx6/
As you can see, on div named "bottom-right" no offset at all is applied. On "bottom-center" it is only applied horizontally and on "center-right" only vertically.
After: http://jsfiddle.net/simon_seyock/sL2f3nL9/2/
All offsets are applied correctly, you could draw a straight line through the anchor points of every div.
