This happens with me too. I discussed this with my speech therapist and what he said made sense. It is because there is no substituting the words in a predecided text. So your mind anticipates blocks way ahead of even when you have to read it out. Say for example, I stutter more with the letter T, so while reading my mind would see the word Tiger coming up and it would go into the stutter mode, fight or flight more or less. Now compare this with a normal speaking situation, where you can easily substitute words, there is no fixed pattern of speech, your thoughts are being spontaneously converted to speech without much effort. It makes a lot of difference. That is the reason why most stutterers always stutter when they have to say their name or say any information which is fixed/static in nature. There is no scope of substituting your name Jimmy with Alex. You may get away with saying Jim for a while, but soon you'd start stuttering on that too. Our mind already has negative emotions associated with these pieces of information, like our name, mobile number, etc., so whenever a situation comes up where we must say this information, our brain associates the present situation with a negative experience from the past where we were not able to say it properly. My 2 cents