Technically it all depends on what is contained within a track, since the bitrate is just an average overall for the whole song.
All interesting for sure, but the main issue here is that the OP seems to assume that when a flac file is reported to be 300kbps, it would be overkill to encode it to 320kbps mp3.
It isn't, since the flac is lossless, and converting it to mp3 will by design degrade it. Always.
This isn't about actual technical bit rates, it is about a misconception that bit rates of lossless and lossy codecs are to be considered equal and comparable. They are not.
On a side note, not relevant to the actual q and a here, I am curious about the <320kbps flac files the OP seems to have in his library.
Are they mono recordings, audiobooks, what?
Or as psychoadept suggested, possibly defective files?