> for reasons that I don't really understand.

I believe it's just the amount of time it takes to physically write a sentence with pen and paper, compared to spoken word or even typing which can be pretty fast in comparison. Your brain is mulling the words over several times over as you write it. That leaves more of an imprint as you are literally thinking about it more as your hand slowly writes each word out. I can type pretty fast, but I don't remember what I type nearly as well as what I physically write out. I just think it's the speed difference and how much time you toil with the specific thought.