RSD uses MAC address as its primary matching mechanism. Do the affected machines connect your network via VPN? VPN clients will often report the same MAC address (the MAC address of the VPN concentrator rather than the actual MAC address of the client) so that could explain it. You can change how RSD matches clients in the Menu | Configuration | Server Settings | Detected System Matching page of the EPO console.
also check out KB52949 which could be helpful...
KB57886 describes the logic behind this as well.