I am assuming that you have 'Verify Database'...this usually will update the datatype...especially if the sp has changed since the report was created. Another item to check, is the field in the sp a float. Just recently, I selected NULL for a field and did some processing on it later, and all my decimal values became integer...I actually needed to Convert(Decimal,NULL) to get it to work...sometimes SQL Server is odd that way.
HTH
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