Actually, as a native Berliner I'd never use spontan when making plans. I guess this usage might be the result of talking about the future the way you'd like to

Actually, as a native Berliner I'd never use spontan when making plans. I guess this usage might be the result of talking about the future the way you'd like to reminisce about it once it's become the past. "We spontaneously decided to sit down at a café to have some jelly doughnuts." sounds more interesting than "We couldn't be arsed to decide beforehand where to eat", IDK? But it's still understandable, same as when people call jelly doughnuts something other than Pfannkuchen, so no big deal.