Difference between Pingdom and Webpagetest

I've noticed big differences between pingdom loading times and webpagetest. When I use Amsterdam for both, and best connection for webpagetest I have for example 1.2 sec in Pingdom and 2.5sec in webpagetest.
Any idea why there is so much difference?

Other thing I don't understand with pingdom: sometimes when i run the test, it last for 5sec with the counter goind up (2sec,3sec....), then it shows 1.3sec as a result... anyone experienced this before ? I'm glad pingdom tells me this slow number but doesn't seem to be relevant as people tend to find the website slow.

nobody has experience with these 2? trying to optimize my site the best I can but need feedback to know which one I should trust more in order to make the right decisions. Thanks guys