If you use the Excel spreadsheet as a database source to test how the field is interpeted by Monarch what sort of field do you get? Numeric or character?
Are the fields the same size?
Do they have the same alignment?
You may need to apply a function or two to the import field to make a match on numerics - or use hidden calculated fields maybe to ensure both values are the same (Character?) and compare on that.
Let us know what you find.