I was clear ahead approaching the mark — why did I have to give room?

Rule 18.2(b): if a boat becomes overlapped before either boat reaches the zone (three hull lengths from the mark), the outside boat must give the inside boat mark room — even if the outside boat was the leader on entry. World Sailing Case 2 is the canonical version.

<iframe width="560" height="315" src="https://www.youtube.com/embed/QGHb8kq9D58" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/leeward-mark/case-2-leading-into-the-mark/">Source: Sailing Kiwi</a></p>