The code I have there is [CENTER][QUOTE]Test.[/QUOTE][/CENTER]. As you can see the center alignment BB code is outside of the quote BB code, and yet, while the text is centered, the quote block itself is not.
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.