Absolutely. As stated that is a calculation based on all previous difficulty changes.

You might find this useful - http://www.google.com/ig/directory?url=pastehtml.com/view/b1cx89lyr.html

As you can see, only the last 3 difficulty changes are significantly statistically different from the historical average.

I might well add a drop down next to difficulty change textbox, covering the options from the gadget, if there is sufficient demand for it.