You are erasing a file which was marked as compressed by NTFS, not just a zip file. These files are also compressed at the file system level. Because of the way the data is stored, Eraser cannot guarantee that the data is cleanly removed and as such refuses to erase them.
To get rid of them securely, you'll need to delete the file permanently and do an Unused Space Erase. While v5 had this feature, it was buggy and corrupted quite a few drives. Not to mention that it wouldn't work with Vista+ with UAC enabled...
No, I am not using file system compression. Please see the attached screenshot.
In Folder options dialog you can see that encrypted and compressed fiels are shown in color. In explorer window you can see that diffutils-2.0.7-1bin.zip is not shown in color.
In Advanced Attributes dialog for a diffutils-2.0.7-1bin.zip file, you can see that compression for this file is disabled.
In Log Viewer, you can see Eraser reporting that it cant delete compressed files.