Why can I read a book aloud with almost no stammering or stuttering?

Why can I read a book aloud with almost no stammering or stuttering? The title is pretty self-explanatory. I know this is the case with a lot of people (sadly not for everybody), but when I read books aloud I will go from a normal 5-6 on a scale of stuttering to about 1 or maybe 0 if I’m feeling on fire. Is it the anxiety that comes with being social that causes us to start hitting blocks? This is very interesting to me and could be a bridge between A pretty mild stutter & nearly not stuttering at all. What do y’all think?