To wager a guess, those additional bitrates beyond 320 were included so that the compression size as displayed in the main window made more sense when using lossless codecs.
Unless you use the %r parameter in EAC, the bitrate is meaningless when the parameter passing scheme is set to "user defined encoder" (as it should be!). If you try to pass a bitrate higher than 320 to lame, you'll probably get an error. Feel free to try it and tell us what happens.