CCleaner seems unable to overwrite files that are compressed under the NTFS file system. Usually, if I clean a large amount of data, CCleaner will take a while to overwrite all of the files. However, if the files are NTFS compressed, the files are simply deleted and no real time is spent overwriting them.
I'm not sure if any data is actually overwritten. But it seems that CCleaner simply deletes the file instead of overwriting it.