Assuming the LR5 gallery is similar to the LR6 gallery...
If you are referring to the image inside the frame, its placement is being done with margins instead of "justification" (left, right, center).
If you are referring the frame, it too is being positioned using margins.
See the file "master.css" for some help. The use of margins seems to make this harder than it should be, but my CSS is a little rusty.