Diffs are only computed when you call it via key bindings, so if you edit the file after generating a diff, the old diff will still be there until you clear it or regenerate it, which can be kinda confusing.
The only way to compute a diff is through key bindings, and the default key bindings are ctrl+k, ctrl+d for diff, and ctrl+k, ctrl+c for clear diff, which will require some memorization or customization.