Download Speed

I have a cox preferred account that is supposed to provide 50 mbps download speed.  When I use speed-test.cox to measure it, I get only ~25mbps while speedcheck.cox gives me 50mbps.  Other online sources (Ookla) measure the download speed st 30 - 35 mbps.  I connect to the modem thru a router, and have replaced the old one with no change in the resulting download speed measurements.  I am very curious about this........any ideas??