My understanding with your issue is you want same material variant should be selected even if few characteristic value differ from the characteristic setup for material variant.
It can be done by setting exclusion of those characteristic. Please check below link in which i tried to explain how to setup exclusion. Go through it and reply with your findings.
http://scn.sap.com/docs/DOC-57122
Please rate the document if you find it useful.
Thanks
Ritesh