Speedtest results for Gigablast - what is expected?

Running Cox's speed test I get on average 260-290 mbps download;  on gigablast.  Is this normal? What is the expected normal day to day range I should see? 
     Surfboard docsis 3.1 modem
     Netgear Nightawk router
     Speed test does not vary much bypassing router but I haven't tried in a while.


From a customer's perspective if my daily average is 300 mbps; why not lower my plan to the 500 mbps plan if I never reach higher speeds? 

Is there something I am missing or am I on the right track here.