So... Sonic CD has Sonic 1 skin, showing it hapened before Sonic 2, want more proof? The lack of Tails and Sonic's plane is also proof.

And you'll say "wait... sonic CD as a metal sonic better than sonic 2", I'll ansewr, you aren't fighting Eggaman from sonic 1, you are fighting modern eggman, because he looks less round and with more human appearence, looking less like an egg and more like a fat man, like in the modern era, and modern eggamn has a perfect metal sonic replic and would know how to dominate litle planet, abusing the main purpose of the game, TIME TRAVELLING.

So basically:

-Sonic 1 happens

-Then comes Sonic CD, where a future version of eggman comes to the past (where sonic didn't knew most of his abilities) in order to defeat sonic before he defeats him.

-In Sonic 2, Future Eggman isn't there, but the past eggman that was there saw metal sonic in action, and got inspired to do a metal sonic, that took a long time to perfect, until generations, that for me happens after CD (you mess with time multiple times and wake up a time devouring entity, makes sense to me), where classic and modern eggman (the one from Sonic CD) and share secrets on how to build Sonic, ansewring why does Metal Sonic in Sonic 3k look so much better than Metal Sonic in Sonic 2.

You'll then say "what about silver Sonic?"... The master system timeline is not the same as the genesis/normal timeline.

But hey! That's just a theory, A GAME THEORY!

(let's hope Sonic Origins agrees with me)