It sometimes suggests a route I don't normally take, then when I continue on the route I normally take, the ETA updates to say I will now arrive sooner.
Google Maps intentionally suggests to take longer routes.
It could be that Google Maps is saving everyone time by doing this. The Braess Paradox is really interesting, and this could be an attempt to prevent it: http://vcp.med.harvard.edu/braess-paradox.html
However the interesting thing about braess paradox is that even if you keep the number of cars constant, introducing a new road can make travel times worse.
It usually suggests three routes to me, each with a time estimate, and the highlighted one doesn't always have the least expected time. I figured it might be respecting my other preferences, but it could equally be trying to balance traffic across the whole network. Though this also happens when cycling, which has a minimal impact on congestion.
It sometimes suggests a route I don't normally take, then when I continue on the route I normally take, the ETA updates to say I will now arrive sooner.
Google Maps intentionally suggests to take longer routes.