Does anybody know why the software does this?
Depends on the software and the script you used. For example, if you put something like "The number plate should read XXX 123", you'd be depending on the AI engine recognising what a number plate was and being able to create a custom one, and in this case I think it's done pretty well. All the engines are really doing is constructing a composite image based on a massive image search using your script.
Nice image though.
