The numbers are just sequential.
The most common types of UTP cable are:
Cat5 - bandwidth up to 100Mbps, signaling rate of 100 MHz
Cat5e - bandwidth up to 1Gbps, signaling rate of 100 MHz (up to 200); basically Cat5 cable that passes more rigorous testing
Cat6 - bandwidth up to 1Gbps, signaling rate of 200MHz (up to 500); preferred cable for Gigabit Ethernet networks
Haha, that was late