I don't have a WiiU, but my guess would be that 480i is the default main output resolution; this is VERY common for compatibility reasons. 480i has historically been extremely widely supported, so a lot of things default to it so that you can always interact with your console even if options get messed up (for instance, the original Xbox menu always outputs 480i even with YPbPr cables, and Xbox 360 component cables have a 480i switch right on the hardware).
The signal itself wouldn't be interlaced, but the progressive signal being sent to the gamepad might just be a de-interlaced 480i feed instead of true-rendered 480p.
That would be an extremely weird content output pipeline. Instead of just merging fields before output, you'd be scaling then splitting then merging then outputting. Merging when the game natively generates as fields makes sense as a simple solution, this seems like it would likely take more work on a system design level than the obvious "correct" approach.
An FPS game, since it gives you the fine-tuned camera control to gauge combing. Or if you look at the sides of objects in a racing game. Or something. Since the gamepad likely wouldn't try to cleverly deinterlace content in a progressive stream, just about anything should be blatantly obvious.
What about Wii mode though?
Can't see much different myself, other than maybe a slightly different animation frame.
However, don't use Miiverse to upload, go to abload.de instead and upload from there. It should preserve at least some of the quality.
Like this.
That's not the same as inventing designing an entirely new and backwards display type though
