I thought I'd pinned it down to the "share this" code (the share buttons). Once they loaded, the problem seemed to ceases, and they didn't load until you scroll down a little, but even with the share bar removed, it does this "not centered until you scroll down a little" behavior. It's like some centering code isn't taking effect until you scroll down a tiny bit. Once you scroll down, even a tiny bit, everything centers, and stays there.
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