Exception message overwriting the variable in the pipeline

How come the exception message that by default an html format that BC sends to sending partner when an error occurs in the service using the EXIT on FAILURE somehow overwrites a predefined XML output template variable set in the pipeline? All variables defined in the pipeline are not visible in the output although in the output template %value errorMessage% are set up, only hardcoded text defined directly in the output template can be seen on the output.