I had this same question. I don't think anyone is going to be able to answer because I think the answer is probably political bickering behind closed doors. Productivity is open to interpretation and it determines who gets money and who's boss gets money. The perception of productivity is at the core of politics at a place like Google.
I was actually wondering the same. Maybe they have to build X number of features of fix Y number of bugs?
What happens when someone is underperforming?
At one company, my boss got talked into trying to count LoC so that the combined QA testing / customer support organization could charge their costs against each development team in proportion to their code size. Our team spent the next week cleaning up and removing code left and right. I think we easily chopped away more than 25% of the lines in our codebase. After the first weekly LoC total report, that effort faded away.
I think the only time I've felt like I could actually show my boss any measure of productivity from our teams is when we were trying to follow SCRUM reasonably closely and it was easy enough to show who had allegedly completed what during each and every sprint. I did have to juggle to make sure that each person showed enough hours, which was a pain because we'd assign a task to a single person no matter how much collaboration it took.