Ask HN: Google PageSpeed Insights Speed Fluctuation

I've been playing with the PageSpeed tool here: https://developers.google.com/speed/pagespeed/insights/

I'm testing a variety of pages and have my speed score for specific pages fluctuate. One page goes from 77 to 90 to 78 to 91. It's quite odd. I leave a few minutes between each test as to make sure that each run avoids the 30sec cached results. Anyone else experience this? All the stackoverflow and google forum replies seem to provide incomplete explanations.