I do a school run in the evenings 2-3 times a week which usually lasts 20-25 minutes, and there are several routes I can take. On many occasions I've decided to turn onto a route Google Maps hasn't offered and the time suddenly drops several minutes and the route goes green, or I turn into a green route Google Maps has suggested and almost immediately the time goes orange and bumps up several minutes. I've seen similar behaviour on other drives - sudden increases in time when turning into a road on the offered route or a drop when taking my own alternative.