I need to know a answer to this...

So, i was thinking about the mainline Sonic timeline, and i remembered Shadow Generations begins with Sonic Generations but ends earlier, so this means we can say we are in 2011, and there is SOMETHING WEIRD ABOUT THIS, how the frick does Shadow Generations have Sunset Heights (from Sonic Forces, 2017) and Chaos Island (Sonic Frontiers, 2022), stages from future games?

I know the game is about different dimensions and different possibilities, but, idk, seems weird