How did this item get in 'Notification Center' and how do I remove it? I have something called 'Pens Labyrinth' in my Notification Centre (screenshot below) -- How did it get there?  And how do I remove/delete/uninstall it?

(Also I don't understanding why am I'm seeing notifications from it while it's in the "Not In Notification Center" list.)
It got in there through a popup like the following:


You can remove it from the list by going into Safari, opening its Preferences, and managing through the Notifications tab.

It seems that, at least on Mavericks, if you have the Notification Style set to Banner or Alert then it will force the notification into Notification Center.