These speed tests arent neccesarily indicative of Cox's performance. If you try repeatedly on different servers and still cant seem to top 2000-3000kbps you may be having some issues with Cox.
These speed tests vary greatly on the server and what backbone paths it takes to get to them.
The server in Stillwater is closest "as the crow flies", but not "as the light traverses the fibers". For Cox in Tulsa to get to that Stillwater server, Cox goes to Level3, which goes to Dallas, Switches over to Sprint in Fort Worth, then comes back to the Chickasaw server in Oklahoma. Perhaps the lower numbers would indicate Sprint's fiber network isnt performing as well as Level3's.
I just busted this on a Kansas City server, much faster than I got on that Stillwater server.
Of course, to the average internet user, they don't know or care which websites and servers bounce through what networks, they just see the download speeds. All I am saying is that Cox can provide an eleventy billion meg per second connection to you, but you will still be limited to the speeds that the sites you are communicating at can serve you at. I think the faster the speed your internet provider offers, the more pronounced this issue becomes of not always being able to get your "advertised" speed on some sites.