could be anything... ;-)
Is there a file in the folder? Or just an empty folder? If there is a file, the file most likely did not match anything valid in the database and was not rebuilt/removed....and so the folder is not empty and won't be removed, either. Then the question would be why a previous scan did not report the left over file as unneeded.....
Of course there is always the operating system which may lock a file / folder and when cmpro calls a remove file/folder function it simply fails due to locking (e.g when a windows backgroundtask accesses it..(virusscanners etc..).
When using network drives it can also be a (temporary) network problem......there are known SAMBA issues for example where files are simply skipped (due to outdated drivers etc..).
So...'can be anything' ;-)
So....what you can try is to simply rebuild the left overs again to see if they get cleaned up. Check if you got the rebuilder advanced option enabled which is reposible to remove empty folders...
I will do some tests here, too....later....Happy New Year
*update * ok...I can repeat the behaviour.....interesting...very interesting.....