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I think it is too unclear what exactly this is. I thought it was actually a joke at first, as if it were facetiously claiming that many of Google's projects are following in the footsteps of others.

Once I realized it's essentially a different theme for Bootstrap, I decided it's actually pretty cool, though.

That's really cool! I especially like the Google bar component.

And what are the main differences with Twitter Bootstrap?

Check the components

no differences functionally. it looks to be a theme, similar to the ones available at bootswatch.

from the projects readme.md:

Overview Because I'm a fan of the new Google UI seen in Gmail, Docs, Calendar, etc, I decided to reproduce the look of these new UI elements for my own personal use.

Seriously, it should be called "Google-Style Bootstrap Theme". The title makes you think it is some new project.

Google-styled Bootstrap