This is mostly technical. The quote/code/etc blocks actually take up 100% of the width -- they just have a fixed right margin (well, in most cases). As such, strictly speaking, there isn't anything to center. If they had a defined width, then it would be possible to do it, though it would need extra CSS (as text-align wouldn't touch them).
So I think this is a combination of a few statuses: partially as expected, partially a design/browser issue due to possible CSS limits.