I don’t understand calling John von Neumann “John” early in the article and then switching to “Johnny”throughout the rest of the article. To me it seems disres

I don’t understand calling John von Neumann “John” early in the article and then switching to “Johnny”throughout the rest of the article. To me it seems disrespectful. I don’t understand why it was done. What did I miss, if anything? (As an aside my first name happens to be John.)