download speed (missing 20mbps!)

hi, i have the cox premier internet package. my download speeds are supposed to be at around 50mbps i would like to know why i am only getting 30-32mbps? i have tested this with 2 different routers ($150-$200) my current router is a net gear R6250 Smart WiFi router and my modem is a motorola surfboard model number SB6121, it's also a docsis 3.0 if that makes a difference. when the cox guy came out to install our service he had us test the speeds and it was only 31mbps and I've tested it about 20 times since then and its never been over 33mbps. he said that it may have to do with our coax cable which i think is some bs. i should not be missing 20mbps if I'm paying or a 50mbps service!