Slow speed test on Cox Server but fast on LA servers.

I used the Cox Speed test on their website and the result indicated my ultimate internet speed was about 70 mbps.  Way slow as it was supposed to be at 300 mbps.  I measured this repeatedly and got similar results at different times of day.  Then i used the Ookla Speedtest website and used the LA servers and got speeds around 300 mbps which is where the speed is supposed to be at.   What gives here?  Was on the tech line with Cox before i realized that my speeds were much higher on the LA servers.  Cox recommends you do the speedtest on their website which uses their servers.  So why would the speeds be so much lower using their servers?