My writing is much more formal than speaking. If one writes much enough in a certain style, they get used to it. Same thing about speaking - politicians, actors, and other public figures have a certain effect on their everyday demeanor.
Once I heard that writing and talking involve different areas of the brain.
I'm always super curious about how someone's method of learning a second language impacts the way they use that language. My casual hypothesis is that what you read while learning impacted how you write, while what you listened to affected how you speak -- do you find that to be true?