This is Apple’s extra thing. As the invitation clearly puts it, “we have a little more to show you”. The use of the word “little” could be the key for the iPad mini (as Apple usually leaves hints like these in their invitations), but it isn’t a certainty, just a high possibility. Also alongside an iPad mini could be a 13-inch Macbook Pro with Retina display.
Whatever it might be, it will be unveiled on the 23rd, at 10am PST/1pm EST at the California Center in San Jose, California.