I analyze my sites straight from
Google. This gives me a better idea than using other tools such as Pingdom, etc. as it gives me constructive feedback of what Google wants for brownie points in their algorithm.
XX/XX refers to Mobile/Desktop - I think I've only seen one website with 100/100. That's nearly impossible as you need to get below a 200 millisecond latency and DNS alone can take more than that, with Anycast and a CDN.
Mobile can be lower because it can be picky with how links are closer together and it may give the end user a bad experience as they're trying to click "About Us", but it keeps going to the "Contact Us" page (right next to it in the navigation).
There are several other factors than just this that are analyzed on both platforms. I believe that even though they don't let you select a server or device to test from, they are doing it from specific locations and devices as well. I've came to that conclusion because you must wait 30 seconds between tests, I see the Googlebot using different user agents accessing my site and the scores will bounce around (+/-), even though nothing has changed.
A typical minisite will have a 96/98 score after everything Google suggests is corrected, and that's because it's out of my control. Google wants me to leverage browser caching. So, I did. However, running it again, they want me to leverage browser caching on AdSense and Analytics. In other words, they dock you points for not doing that (which you could, but would be against the TOS).
Either way, I see my position in the search engines higher following their constructive feedback and fixing everything (in my case, doing it once and automatically using an Apache module mod_pagespeed).
From my first test in the mid 60's/70's to 98/99, I saw a 1 to 3 page jump once crawled again for certain keywords. I remain in the same spots.
It leads me to believe that Google is putting Page Rank high on the pedestal as it does give the user the best possible experience. Everything they see loads first and quickly, while what they won't see for another second is still rendering.