From other comments, it's obvious that 1) Google expects you to do your 100%, and then a 20% on top of that of "Google related projects", most likely owned by Google, for free.
What happens is that some managers really don't buy into the 20% concept, and don't provide time for it. Employees in those teams determined to do it anyway end up doing 120%.
There is absolutely no requirement at Google to put in time on a 20% project.
That said, what happens in practice is that 20% projects are the way that Google engineers are able to move across teams: You pick a team you want to join. You do something for that team as your 20% project with the goal of getting that manager to request your transfer to their team.
So if you're desperate to get off your team without quitting Google, you could get backed into committing 120%.
https://en.wikipedia.org/wiki/20%25_Project
Former Google employee and Yahoo CEO Marissa Mayer once stated “I’ve got to tell you the dirty little secret of Google's 20% time. It's really 120% time.”[6]
https://www.businessinsider.com.au/google-20-percent-time-po...
Yahoo CEO and formal Googler Marissa Mayer once bluntly denied its true existence.
“It’s funny, people have been asking me since I got here, ‘When is Yahoo going to have 20% time?'” she said on stage during an all-employee meeting at Yahoo. “I’ve got to tell you the dirty little secret of Google’s 20% time. It’s really 120% time.”
As a current Googler, I've been spending 20% of my time on self-directed work, and 80% on what is assigned since I started, and my contributions have been to both open/released software and also internal stuff, and I joined in 2006.