Well, I'm sure people feel that way about my freewrites quite often, but that's just how my mind works. I don't think ahead and I just write. My husband says quite often, "If I hadn't just watched you bang that out, I would NEVER believe that you just did that on the fly."
Of course I've also done a few 'live' freewrites at open mic nights (either singing to blues music played by the band or just storytelling) with prompts from the audience and it always comes out pretty well too. I think some of us just have more improvisational brains. I for one SUCK at writing anything that is planned out. It always sounds choppy and fake to me.
RE: Five minute Freewrite: "Artichoke"