I think a lot of it has to do with your interests. For example, you're a HN reader and you come away with "Yellowstone == supervolcano." A new mom would probably read the quoted text and think "Yellowstone is dangerous, don't take the family there." A kid who is into fireworks and explosions might remember "TNT the size of Rhode Island, cool!"

When I read, I usually just remember the big picture plus the small details that I personally find fascinating.