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#11
This problem has zero to do with cables or speed tests. What your set speed by AT&T in http://192.168..254/cgi-bin/broadbandstatistics.ha for Downstream and Upstream are what you are getting from AT&T. Every machine is going to be different if something is happening on the network at the same time and on those computers. You can get 1000mbps in the network adapter settings and your set speed by AT&T are correct. That has zero to do with the fact that speed test websites are not meant to know what your ISP allows you to have. It is what you are getting between your computer and the speed test website, which operates like our Interstates and highways. Speeds will vary.
Make sure that you are using the AT&T servers if you test from speedtest .com. Always use closes server. Your speeds will vary. Every OS is going to show different speeds, especially computers with onboard network adapters.