Speed shows 100.0 Mbps but why not 1.0 Gbps?

Question

I have a very simple setup. A Verizon Fios Actiontec MI424WR going to my Realtek RTL8168C(P)/8111C(P) Family PCI-E Gigabit Ethernet NIC. For some reason my speed is shown as 100.0 Mbps. Why doesn't it show 1.0 Gbps? The cable?

Answer 1

Quoting directly from The Manual (for the latest MI424WR Verizon is shipping):

LAN Ethernet Ports (4)
The LAN Ethernet ports connect devices to the FiOS Router via Ethernet cables to create a local area network (LAN). The LAN Ethernet ports are 10/100 Mbps auto-sensing ports, and either a straight-through or crossover Ethernet cable can be used when connecting devices to the ports.

The LAN ports are NOT gigabit - they are 10/100.