The lease isn't being offered by the Charter DHCP servers. It's being offered by the cablemodem. My guess is you've exceeded the maximum number of CPE's that can be behind the modem.
Let me make sure I've got your configuration right. You have a couple of PC's connected to a router, and the router and the PIX are connected to a hub or switch, which is then connected to the cablemodem. In this configuration, the router would get it's IP address from Charter's DHCP servers, and then would serve the PC's internal IP address to the PC's. If the max CPE for the cablemodem is set for 1, the PIX would not be able to get another IP from Charter, and the cablemodem would default to an attempt to serve a short-leased, temporary IP in the 192.168.100.x range.
Or maybe, since you've said the PC's are getting their IP's from Charter, they're not behind the router. What's connected to the router then? What every is behind the router would get an IP address from the router, not Charter.
Whatever the setup is, the most likely reason for the cablemodem to be attempting to serve the address instead of the Charter network would be that you have too many devices already directly connected to the modem.