It's pretty common for post titles to be changed (by HN admins). The most frequent change I see is they change the title to match the title of the original linked article/story/whatever, which seems to be the case here.

Why, I can't speak to the reasons in this case but I suppose when you allow any user to make up anything for a title, they sometimes inject their own opinion or otherwise make the title not properly reflect the intent of the original title. Not saying this is what happened here.

Of course it can go the other way. Sometimes original titles are not clear, and the HN title can get preserved or changed for clarity or neutrality.