Fast Tx rate, slow wifi tests

I am paying for 600mbps speed and only receiving 150-200mbps. The Comcast guy came out and tested my internet. The line into the house is sending 1,000mbps. The router is sending 623mbps to his machine LAN tested. My txRate on my MacBook wifi is 650mbps, but the speed test reflects less then 200mbps. Why is my tx rate appropriate for my subscription, yet the speed test results in much lower speed? How can I fix this?