You're assuming a lot of stuff again! Now, let me tell you some things about coding UI because it seems like you don't really understand how this works, particularly because you consider it bad coding apparently.
First of all, I'm not sure if the developers know how many more stages they will have. They probably didn't at the start of the DLC development, though they might do by now. Thus, they were not able to program the amount of "stage slots" (I'm gonna continue calling those slots for all purposes here now) in advance.
Furthermore, DLC stages aren't simply stacked onto the game like some sort of Lego. The stage has to be implemented into the code, including the UI. All of the UI's aspects have to be manually included. The game itself can't code the pointers for music, preview images, etc., this has all to be done manually. That includes rearranging the selection screen, unless they put in placeholders for additional stages. That's why the stage selection screen got rearranged with every stage that has been released. They rearranged the slots manually to fit the new stages. If the DLC stage screen selection was built for 5 stages, it can't just magically hold 6.
Of course the game automatically detects which stages you have and will arrange them properly for you depending on how many you have, but that was also put in by the UI programmers. They coded the game so that it would display those stages in orderly fashion depending on what you own because that's what they wanted it to do.
The game itself can't create code that makes a stage appear in the selection screen. I have no idea why you think that a game not writing itself is "poor coding".