You wonder why the exception/error is not appearing though all the criteria you have set is matched.
SAP Business ByDesign
Default criteria : Same Supplier, External ID matched 80% or above and Invoice date + or - 7 days.
Additional criteria :what you set in BC activity "Supplier invoice exception "
In general ,when all or one of the elements in default criteria does not match then system will check for additional criteria.
however system will also check for one more criteria that is PO reference matched or not when additional criteria is checked.
so in your case PO reference is not there or is not matched and thus no error is appearing.
possible Duplicate , KBA , SRD-SRM-SI , Supplier Invoicing , Problem