The PIX-501 has a 4 port Unmanaged FastEthernet switch that is connected to the Inside interface via
an internal port. In PIX versions 6.2 and below, this port was hard-coded to 10 Mb / Half-duplex. In PIX versions
6.3 and higher, this port is hard-coded to 100 Mb / Full-duplex. This setting cannot be changed. However, the 4 switch ports are set to autonegotiate speed / duplex, and cannot be hardcoded.
In short, the 100full covers the internal port that the 4-port switch conects to, and this can't be changed. The actual speed-duplex on the 4 external ports are set to autonegotiate and this also can't be changed.