Why do people say there are 51 states in the US?

I've noticed that some people claim there are 51 states in the United States, which contradicts the widely accepted fact that there are only 50. Could you elaborate on the reasoning behind this belief? Are there any specific factors that contribute to this misconception, or is it simply a matter of misinformation spreading among the public? It would be helpful to understand the origin of this claim and why it persists despite the official number of states being well-documented.