For the same reason we stutter worse when under stress. Or the same reason people with something like aphasia or Tourettes can experience their worst symptoms when under anxiety. It doesn't mean it's all in their heads. It just means the energy we're not expending on communication, insecurity, stress, excitement, desire, anxiety, pressure, etc and so forth, can be used to overcome our disfluency. People really underestimate the canyon of difference between speaking to another person and speaking to no one. A lot of brain resources are tied up in communication. For some stutterers, speaking to no one allows them to achieve fluency. For others, it doesn't. That's the difference in how we stutter, but not a fundamental difference in us *as* stutterers. Like how some people are fluent while reading aloud, but some will experience their worst disfluency while doing the same thing. Are they different stutterers? No. They just have differences in how they do it. At it's core this is a question of what is the exception and what is the rule. People like to think the stutter is the exception, that when they're speaking to people and they stutter, this is not how they're wired, it's a fluke. In reality, it's the other way around. Your alone-fluency is the fluke. After all, we speak to others far more than we speak to ourselves. And everyone experiences anxiety, stress, pressure, and difficulty communicating. Only *we* stutter.