HAMMER:hammer prune-everything's problem

[lhmwzy]

My file system  structure:
/www /www/mirror
When I delete files from /www/mirror and run "hammer prune-everything
/www",it does not free space for /www
Only free space by run "hammer prune-everything /www/mirror".
Is that a problem?
I think "hammer prune-everything /www" should free space for any sub
file system on /www.