*sigh* That's because Dimps are making the game and Sonic Team is simply overseeing the project, exactly as happened with the Advance and Rush series as well as Sonic 4.
It's probably the same case as Sonic Unleashed Wii, where Sonic Team was developing the Werehog stages while Dimps developed the day stages.
http://www.sonicstadium.org/2011/05/nintendo-power-sonic-generations-3ds-details/
This says co-developed, but you could be right, as Sonic Team usually takes a little more credit than they deserve when it comes to Sonic games made by Dimps.