Javascript window.open tab/new window, trusted sites.

I have IE10 with the following Tabs settings:

When a pop-up is encountered: Always open pop-ups in a new tab.

Open links from other programs in: A new tab in the current window.

I then have a test HTML page where I try to use the Javascript window.open function to open a new tab:

window.open("http://www.bing.com");

However, this opens a new window, how come? If I replace http://www.bing.com with something in my trusted sites list, it opens in a new tab. If I remove it from my trusted sites list, it opens in a new window again. I can't find anything about this behavior.