It's pretty weird that with the same HTML in place, IE9 is opening some links in a new window but others in a new tab.
The below ones are being opened in a new window.
<button onclick="window.open('http://www.gmail.com', target='_blank')" type="button">Gmail</button>
<button onclick="window.open('http://www.wikipedia.org', target='_blank')" type="button">Wikipedia</button>
And the below link is being opened in a new tab.
<button onclick="window.open('http://www.google.com', target='_blank')" type="button">Google</button>
IE9 Version I am using : 9.0.8112.16421
In the tabbed browsing settings, I have selected "Let Internet Explorer decide how pop-ups should open"
Any help would be appreciated!