Precedence Constraint Expression

SQL 2008 R2

I have a variable, varFileName, that holds the value of the text file I'm importing.  I want to go one way if the value of the var is "claim20131031.txt" and another way if it's a different value.  I setup a Precedence Constraint with this expression @varFileName == "CLAIM.20131031.TXT".  It works perfectly if the file I'm importing is that filename.  However, when I import a file with a different name, the Expression that is supposed to take a different route doesn't work.  That expression is @varFileName != "CLAIM.20131031.TXT".  Why would the == one work but the != one not work?  The var is a String Data Type.

Thanks in advance.

André