Files with similar names (but different file extensions) are truncated, except for the file extension, when viewing file tabs. This helps alleviate some clutter in the File Tabs tool window. This is a new feature in SlickEdit 2008, and it can be disabled by right-clicking on a file tab and unchecking Abbreviate similar files.
For more information, use the menu item Help > Contents, and browse the Contents listing for "User Interface > Buffers and Editor Windows > Managing Windows > File Tabs". (I'm filing a task to add better index markers to this location.)