Why does Document.insertString() auto-scroll with getLength()?

Question

I noticed some interesting behavior with JScrollPane when manipulating the underlying document in a JTextPane. Consider this example:

public class Example {

    public static void main(String[] args) {
        String text = "I like trains :)\n";
        createExample(1, doc -> doc.insertString(doc.getLength(), text, null));
        createExample(2, doc -> doc.insertString(doc.getEndPosition().getOffset(), text, null));
    }

    private static void createExample(int exampleNumber, ThrowingConsumer<Document> insertAction) {
        JTextPane textPane = new JTextPane();
        JScrollPane scrollPane = new JScrollPane(textPane);
        
        Document document = textPane.getDocument();
        JButton insert = new JButton("Insert" + exampleNumber);
        insert.addActionListener(e -> {
            try {
                insertAction.accept(document);
            } catch (Throwable e1) {
                e1.printStackTrace();
            }
        });

        JFrame frame = new JFrame("Test Frame " + exampleNumber);
        frame.setLayout(new BorderLayout(0, 10));
        frame.add(scrollPane, BorderLayout.CENTER);
        frame.add(insert, BorderLayout.SOUTH);

        frame.setSize(400, 200);
        frame.setLocationByPlatform(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    private static interface ThrowingConsumer<T> {
        void accept(T t) throws Throwable;
    }

}

This creates two frames that append text to a text pane's underlying document when a button is pushed. The frames have almost identical logic, with the only distinction being that the first frame uses Document.getLength() when inserting the text and the second frame uses Document.getEndPosition() and Position.getOffset().

Somehow, the first frame will automatically scroll to the bottom when the text is appended, while the second frame will remain at the start of the document (unless the user drags the scrollbar manually).

Does anyone know why this is?