I see this often on networks where the Ethernet Adaptor is set to 10/100/1000 auto negotiation but has, for whatever reason, decided to limit itself to 100 mbit/s. This could be due to length of the run, the noise on the wire or simply that the adaptors at each end auto negotiated that speed. The first thing to do is make sure that all devices are set to 1000 mbit/s on their ethernet adapters and then rerun the speed tests.
You should never do a speedtest using wifi in any form, as that will always yield lower performance than a physical 1000 mbit cabled ethernet connection. To truly test your broadband speed you need to do the test directly from the router, or at least a PC connected with a 1000 mbit/s ethernet connection connected directly through the router itself.
