Google has special compensation structure for some startup-like projects designed to keep talent in the company instead of leaving and building their own company. After they hit (or were thought to hit) all milestones, self-driving unit was valued at 8B by independent assessment. As one of the leaders, Lewandowki’s share was 120M.
Also, they bought his startup (510 Systems) to bootstrap the self-driving car unit. It's possible at least some of that $120M was an earn-out from the acquisition.