Hummingbird - Response Time Graph

I am somewhat confused by the graoh that is output by Hummingbird that shows site response times.
Hummingbird say that my average response time is around 7 seconds, but when I go into Google Page views, it says that my server response time has passed the test and when I go into Pingdom it says that my site is responding in under 2 seconds on most pages on my site (including loading all of the images, CSS and everything else).
Could you please explain this wide discrepency between Hummingbird and these other, supposedly reliable, resources.
Is the Hummingbird graph of any value?