Cox Ultimate - download speed help

I have the Cox Ultimate tier and am getting download speeds of 80-85 Mbps after running the Internet speed check, but no where close to the 150 Mbps that is advertised for this tier.  I know the 150 Mbps speed is the upper advertised limit but I would have expected download speed to hit at least the low triple digits for this tier.  My upload speeds are in the low 20 Mbps range which meets the advertised speed but my download speeds are a different matter .  My modem is the Motorola SB6141 and the modem is connected directly to the computer.  I hope you can help.