[vorbis] How are nominal bitrates calculated?

[Kyung-Joon Lee]

When you encode stuff with oggenc, the nominal bitrates (in kbps) for 
-q1 to -q9 are multiples of 16. But the nominal bitrate for -q10 is 
498kbps, which doesn't get rounded nicely.

As I don't know how to read or write code, I have no choice but to ask. 
What's the reason? How is it done? : )

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