I don't think we've 100% decided on a menu alignment. It seems the traditional method is to try to align the left of the menu to the left it's source. I think OS X tries to center popovers on their source. One thing we've been talking about is making popovers float towards the center of the display. We'll see which one wins
the left alignment has that weird effect on the AppMenu, which is located on the far right, opening a menu to the outside of the application window (if non-maximized).
by aligning popovers to the center of their source, you cover all possibilities of sources position -left, right or center of screen- leaving the onus of the alignment to the popovers arrow, which, like in OSX, had to be movable across the popovers top. I don't know if thats possible, but it would be clean.