Variation in no. of request count at document complete

[pmeenan]

The difference between the two is all thumbnail images so I'm fairly certain it is the lazyload thumbnail images.

Depending on what filters you have installed and enabled pagespeed will use beacons from client-side js to figure out what the visible portions of the page are and use that information to only defer images below the fold. My guess is that the variability comes from that information not always being available (not sure when it gets wiped out or how often it recalculates) but you should probably ask over in the mod_pagespeed discussion group.