SharePoint has a logic to prevent user from using some of the reserved words as folder names (e.g._file, _files, .files). So when you create a new folder with “_files” in the folder name in any document library, SharePoint will insert a “_” at the end.
For example, if your folder name is “Releases_files”, if you create this folder in any document library via browser or code, the result will be “Releases_files_”.
The work around is make sure there is no “_files” in your folder name, such as “files” or “-files” will work just fine. Apparently this “feature” has been around since MOSS 2007 or could be earlier, I did not know about it until this week, when one of our document migration code failed…
Found my answer through this link, http://objectmix.com/sharepoint/357916-appending-_-foldername-ending-_files.html