Sitemaps – Why is Home usually at the highest level?

I have attached an example of Google Plus with the App Screenshot and Sitemap side by side to help explain my question.

The example has "Home" at the highest level.

Why isn't "Home" at the same level as the other Nav links? I understand you land on "Home" first, but that shouldn't matter, it's still on the same level right?

Your reasoning would be much appreciated.

Thanks!