Why can I read out loud to myself and be non stop fluent....

Why can I read out loud to myself and be non stop fluent.... Why can I read out loud to myself and be non stop fluent for basically an hour without a trace of stuttering (while recording myself) maybe stumble over a few words, but not enough to even notice that I stutter....then I pick up the phone and I almost can't stop stuttering....It just seems really weird to me. Even speech pathologist do not seem to have an answer for me on that one...