Why are there "Tony Hawk's Project 8" assets within "Tony Hawk's Proving Ground"?

[Resistiv]

Recently, while going through the filesystems of several Wii games, I found that within Tony Hawk's Proving Ground there are several assets (such as loading videos or title screens) that are clearly from Tony Hawk's Project 8. Better yet, most of the assets are found under a folder named "psp". Why would these assets be left within the game? Is this just another example of leaving unused or old assets within a game for no real reason?



Thanks in advance!