Why does the speed tests differ so much?

Our ISP is Earthlink. We have 10M fiber connection which includes a site-to-site MPLS. I'm so confused about the speed test results of Upload generated by between my ISP and others. (Download results by all tools are almost the same at about 9.7M.)

Look, If I use speedtest.net , speakeasy.net or Verizon speed test, etc, the upload speed is only about 3M. It is quite awful, isn't it? But the problem is, if I use EarthLink's own tool provided at its website: https://speedtest.earthlinkbusiness.com/
I will get the upload speed at about 9M. That is huge difference between 3M and 9M. Can you help me figure out which number is more real? And could my ISP be cheating?
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The sites you mentioned are bottle necked  because millions of people are using the site to test there speeds.

Your ISP test site is likely the most reliable.

Can you post link to you plan? In general 3mb upload and 10mb down would be consistent with most plans.

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Your upload speed to your provider is the most direct test. After your traffic leaves your provider it will compete with other traffic. Also, as joinaunion posted, they may be overused.
The ISP test will show you your (mostly) pure upload speed almost. The other results will show you your actual and real "internet" speed.
Natty GregIn Theory (IT)Commented:
If you want to see the real speed of your internet do it at 3 or 4 am,  in the morning. During regular hours when everyone is on the internet, it will slow between 3-5 megs. And the test depends on if the test site its on DSl, ADSL, Dial up or broadband internet, and how far in internet terms how many hops you are away from your ISP.
CastlewoodAuthor Commented:
I was just using another speed test site http://www.phonepower.com/wiki/How_to_understand_the_Bandwidth_Quality_Test_for_VOIP

and the result supports the one by our ISP.
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