tpg speed test - slooooow

Tested using a wired connection between the modem and the pc. 
PC is Win 7 64 bit with 16GB RAM.
Firefox browser.

TP-Link TD-W9980
Downstream the modem says 4.3Mb/s
Speedtest.net says 2.2Mb/s   HALF!!!!
Why is it so much slower!