Если какой-то особо умный прокси стоит или модные во времена диалапов "ускоритель" интернета, тест еще и не то показать сможет. Там же все по http работает
PS: вот и правильный ответ
Why are my test results outrageously fast? (
http://speedtest.net/qna.php#q24)
This is usually the result of something installed on your computer that is intercepting traffic between your web browser and the server. Examples of this would be security software from ZoneAlarm, McAfee or Symantec. Try momentarily disabling the software to see if it is the source of the problem. Make sure you don't leave it disabled! Any software firewall should work with Speedtest.net if configured properly.