The "unassigned" value you see is MC's pretty-printed message for the user interface that occurs when there is no value available (i.e. the expression evaluates to empty, or a field contains no value).
It sounds like your classification is not in use for some file types that are being displayed. Two solutions: either categorize them, or modify the filter for the display to not show those files. The latter is done via: Tools > Options > Theater View, under Items To Show. Select the particular category you want to modify, and then click Set rules for file display... and modify the search list to not include said files. If you are uncertain how to do this, provide a little more specifics about your classifications.