802.11b has transmit rates of 1,2, 5.5 and 11 Mbps. By setting the minimal transmit rates at a value of 12 Mbps, you are basically denying b rates to be advertised and utilized.
802.11g uses OFDM however which has transmit rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps.
So by setting the minimal transmit power at 12 Mbps, you are denying the lowest 802.11g rates which might have an adverse effect in some situations and can cause unwanted retransmits. In that case, adjusting the minimal transmit rate to 6 Mbps also does the trick as 6 Mbps is not a 802.11b rate.