Sensitivity allows Updatez to use different measurements for different pages to determine if they have been updated or not.
For example, Google News pages return the time of the search, so if Updatez checks a minute later, if Google's search was faster or slower, this time would be different, therefore the page had been "updated", but the only thing that changed was the time it took Google to perform the search.
100% Track every change, no matter how small.
1% The entire page needs to be "new" before an Updatez is reported.
For Google News, I have found a Sensitivity of about 64% to be pretty good.
If you're tracking a competitor or an auction, you may want to set the Sensitivity to 99-100%.