So I think the choice for the positioning was mainly influenced by implied context. The closer the likesBar is to the actual content, the more obvious it is that it relates to the content, and the more use it will receive. By placing the likesBar under the contributors, it becomes more ambiguous -- am I liking the content or am I liking these contributions? -- and moving it lower will have it receive less activity anyway.
Because the contextually best place for the likesBar is immediately after the content, it is placed there, rather than after contributors.
However, the standard ordering of the actionBar and the fact that the actionBar only includes 1 link make it both inconsistent with the surrounding style and in a position (all the way to the right) where it appears to break vertical flow. With the likesBar first, this flow change is less jarring, because the bar can guide your eyes rightward.
There are some cases where content has no content in the contributor block, but the likesBar appears.
And when the content is empty and the only visible element is the likesBar, it looks pretty awful in my opinion, due to the break in vertical flow.

It looks even worse on XenForo 1.x, because the actionBar juts right up against the title rather than the likesBar kind of acting as a border.
With the standard order and placing it nearer the content, it increases the vertical space before the contributor block, which pushes the rest of the page further down ("Like" is on a line by itself). On areas/books/etc this can be a major issue, as the child content can get pushed out of view, so every pixel counts.
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By reversing the order of the two elements, we thought we were able to resolve all of these issues at once, with the sacrifice of minor inconsistency with likesBars for other content-types.
I would prefer to let some others weigh in on something like this before making a change. Vertical positioning of various elements is important.