Found the BUG - Yes this is a bug in 10.5
If SmarterStats has an import error - writing to the file below, it will cause itself to NOT delete TEMP files!!
Stopping service, deleting the 2015-09 folder, starting the service fixes everything
Import Error: 3/18/2016 6:51 AM - C:\SmarterLogs\WebLogs\5\2015-09\spdrsessionlengths.stat.new could not be written correctly