View Post Button Buggy - or is it like that for a reason?

When I click the "View Post" button sometimes it opens in a new window/tab and sometimes it opens up in the same window/tab.

Does anyone know why it does that?

If it's buggy - how do I stop it from being buggy?
If there's a reason, it would be great to know. Perhaps knowing when it'll open up in a new window (and when it won't) will make it less annoying.