Looking at the bandwidth line at the bottom of the waterfall it looks like the page was bandwidth constrained in the WebPagetest test. What kind of connectivity was used in the Keynote Last Mile test (i.e. what speed)?
The cable speed came in at 3.5 seconds which is closer to what you saw in Keynote so my guess is that it was a higher bandwidth connection.
Looks like you have Google analytics on the page, try going to Content->Site Speed->Page Timings->Performance and look at the distribution of load times for your actual users to get a baseline (best if you can look at that specific page and for “New Visitors” in the advanced segments).