I really think it comes down to your mind not being as occupied with worrying about stuttering. For one, you haven't planned out what exactly you're going to say - as you put it, when you're telling something you know, the words come by themselves, so your mind is free to think "ok I will stutter on this word, this will be a block.." etc. Whereas when you have to actually make up the words as you go, you don't think about your stutter as much and therefore dont PERCEIVE your stutter to be as bad. I emphasize perceive, because I've had the exact same experience as you so many times. I've been hanging out with my closest friends with whom I rarely worry about my stutter, and as such I haven't thought that I stuttered as much as normally, but at occasions we've had a camera rolling, and upon watching the footage I get absolutely astounded - what the f, do i stutter that much?! But it really doesn't matter if I do or don't stutter much - as long as I don't worry about it!