The CAP theorem says you can't have all : consistency, availability, and partition tolerance. Since network partitions happen, you choose between consistency and availability.
Google Search: Availability over consistency. Serving slightly stale results is acceptable. Users won't notice if an index is seconds old.
Google Docs: Consistency matters more. Two users editing the same document need to see the same state. Use operational transforms or CRDTs.
Spanner: Google's own answer to CAP. Uses TrueTime (atomic clocks + GPS) to provide globally consistent reads. This is unique to Google.