Real-time Bandwidth Monitor detects the interface speed based on the SNMP information provided by the device. If the device does not provide the required information complying with the defined standards, the speed may not be determined correctly.
Real-time Bandwidth Monitor works with basically two tables: ifTable (OID: 188.8.131.52.184.108.40.206) and ifxTable (OID: 220.127.116.11.18.104.22.168.1). During the interface discovery, the application loads the list of interfaces from ifTable and the speed of the interface is defined in the ifSpeed column (column index 5). If the value for the interface in this column is equal to 4294967295, it is a sign that the app should look for this value in ifxTable, column ifHighSpeed (column index 15). Value in the ifTable.ifSpeed column is in bits per second, ifxTable.ifHighSpeed is in Mbits per second.
My guess is that the ifSpeed is set to 10000000, or there is incorrect speed in ifHighSpeed. You can check on the values using MIB Browser, if you own Engineer's Toolset, or you can get one of the free snmp tools - Paessler SNMP Test, or net-snmp.org.