I think that you're assuming that the number of apps that can be reviewed by a human remains constant over time. If Google hires programmers to do the review p

I think that you're assuming that the number of apps that can be reviewed by a human remains constant over time. If Google hires programmers to do the review process, they can identify patterns, develop new techniques, and build tools to accelerate the process. Additionally, I bet that they're expecting the value of the Chrome store to increase over time, which compounds on the effect above.