声明的变量与硬编码字符串的行为不同

I am trying to set a like parameter into a variable and allow that variable to accept wild cards (mssql 2005). If I use my parameter set to '%' it only returns a partial subset but if I hard code the query to use '%' it returns the full set. Why might the variable behave differently to the string?I am trying to set a like parameter into a vari