Why does Html.ActionLink render “?Length=4”

I’m VERY confused as to why this code

Html.ActionLink("About", "About", "Home", new { hidefocus = "hidefocus" })

results in this link:

<a hidefocus="hidefocus" href="https://stackoverflow.com/Home/About?Length=4">About</a>

The hidefocus part is what I was aiming to achieve, but where does the ?Length=4 come from?