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.
When you read, what percent of the information do you think you actually retain?
I acknowledge that might be the wrong way to look at it though. The unconscious mind probably picks up a lot more than we think and that knowledge enables us to make connections that we previously would be unable to do.
There are trade offs between reading time and reading comprehension. For example you can skim through a short story in one minute and come away with 75%. Or you can read through the same story in 5 minutes and come away with 90% (made up numbers).
If I were taking a test with limited time and a lot of reading, I might choose the speed reading because it's most "cost effective". If I were studying, virtually unlimited time, and trying to retain the information, I would go with the latter. If I were just reading for entertainment, it would probably be something in between (which might end up being slower and retaining less).
I agree that subconsciously we probably pick up a lot more than we realize.
When I read, I usually just remember the big picture plus the small details that I personally find fascinating.