Eggs only contain monsters you don't have.

If I remember correctly, DIB did this. Why the change?

I’m not sure why, but if they did that you’ll eventually get all rare arkadions if you collected most of the commons already. That’s an ridiculously easy way to complete your collection, and they want you to collect them by yourself or at least get them in eggs by luck.

It was random in DIB too, they just made it a bit harder to get exactly what you want.

Especially since the thing spins a lot faster than dib does so you can’t really time it up