Windows 7: using speedtest.net to check speeds

Comcast is my cable/internet provider. I recently upgraded my up/down speeds. I if let speedtest choose it's own server based on ping (which is around 35~40) my speeds are not that great. If I manually choose a Comcast server (ping is 5~10) my speeds are where they should be based on the package I paid for.

Is this due to their "Powerboost" feature? Is there another way to check that will give me a more accurate picture on my speeds?

speedtest.net is not actually that accurate, my isp has their own speedtest also and that is accurate .

try this one and see if it is the same as your isp or speednet
click in the yes field before starting the test

BTW Performance Test

Tried that and my speeds were close to what I pay for.

I've used speedtest.comcast.net and this is the result:
http://stage.results.speedtest.comca.../187792142.png

My question basically is, when you download a file (from any website) what server is used? Is it whatever server has the lowest traffic or lowest ping?