Relation between segment name and message offset, if any?

[Harald Kirsch]

I thought the name of a segment contains the offset of the first message 
in that segment, but I just saw offsets being processed that would map 
to a segment file that was cleaned and is listed as empty by dir.

This is 0.10.* on Windows.
Is there something strange going on with data still mapped but the file 
empty or does the relation between offset and name just not hold?

Thanks
Harald