I’ve been using Google PageSpeed Insights quite a bit recently. There isn’t much information on how exactly the tests are run, which can make it hard to reproduce the results. Then I noticed the user agent strings coming from PageSpeed Insights ( emphasis mine ):
Desktop
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko; Google Page Speed Insights) Chrome/27.0.1453 Safari/537.36
Mobile
Mozilla/5.0 (iPhone; CPU iPhone OS 6_0_1 like Mac OS X) AppleWebKit/537.36 (KHTML, like Gecko; Google Page Speed Insights) Version/6.0 Mobile/10A525 Safari/8536.25
The only difference between these and normal user agent strings is the ; Google Page Speed Insights.
If these user agent strings are what they claim to be, the Insight desktop is using Chrome 27 released in May 2013 and Insight mobile is using Safari from iOS 6 released in Nov 2012.
This explains some of the inconsistency I noticed in comparing PageSpeed Insights results with Chrome mobile device emulation.