Disclaimer : I DO NOT claim to be an expert on this but here is what i think Reading aloud is different from impromptu speaking in a social context, speaking in social context requires much more resources from your brain , e.g you’ll have to gauge other persons reactions , certain time constraints , listening to what other person has to say , and forming a response accordingly, on the spot with preliminary anticipation of what words you will be able to say , which ones you wont , paraphrasing , all of that within a matter of seconds , if not fraction of a second. Doing all of that , takes up brain power , that us as stutterers may as well not be “optimised” to do it as efficiently as other people. Or it could just be muscle memory , something that you have associated stuttering with , based on experiences. Reading absolutely fluently suggests only one thing , which is that you dont lack the anything in the physical apparatus ( articulators , vocal chords etc) but rather some abnormality within your neurology , that is only triggered after a certain threshold , which may be different for different people. That being said many stutterers even stutter while reading. There is much variation in stuttering behaviour amongst us. There is nothing definite that can be said about , provided the complex nature of not only stuttering , but the ability to speak itself. It is a complex phenomenon, to say the least.