For those people that didn’t know, Steam stores application data that includes a list of every game that has ever appeared (and will soon appear) in Steam. Even if you don’t own it.
Not only does Sonic Generation have a registry, but so does a pre-order for a bundle, which seems to include every Sonic game available on Steam. Though seems that the original version will include a free copy of Sonic The Hedgehog (Genesis).
To add to the flame, according to Sonic Stadium, the SEGA blog listed the PC version in a post and quickly edited their post.