Stuttering while alone

Stuttering while alone So this might be weird but when writing my short stories I speak out loud, do the voices and come up with mock dialogue on the fly. I can do this all without a single stutter, block or issue. Now ask me to read off the same thing I wrote with someone else, or record myself reciting it and it’s stutter city. I have a lot of blocks and pauses. Im just curious if anyone knows possible explanations because I find it incredibly frustrating.