I've seen constumers get confused by this so yes Nintendo marketed wrong and it really did seem like the Wii U was just and upgrade. For consumers, the "U" could've just stood for "upgrade", the console looks very similar to the Wii, uses the same controllers just with the "add-on" gamepad, and still plays Wii games (granted backwards compatibility is an amazing feature).
From an Xbox 360 to Xbox One the name sounds completely different, a completely different look and controller so it's easier for customers to see that the XB1 is indeed a different console.