right now at 12p UTC there's a different set of top stories.
I call these Euro top stories. Within the next 2-4 hours USA will wake up and the top stories will change.
Having moved continents last year, I think this is very apparent. European morning stores are the day/evenings of the US, and then like you say, between 2 and 4pm the active stories are more highly variable, as the US wakes up.
This would be really interesting to study. Same with Reddit.