When Google (or any other mapping program) gives you a 'best' route it means 'fastest' (usually) or 'shortest'. It isn't saying 'this is a better route' - unless it has a way for you to set multiple criteria.
Shortest is easy to calculate. Fastest requires estimate speed on various types of road. Those speed estimates can vary in quality. In some cases it has enough information to base the speed estimate on traffic conditions (but for when, right now, or next Sunday at midnight?).
With Google Maps you can check the quality of a road by zooming in. With Streetview you get a 360deg view at road level, giving the clearest picture of whether the piece of road is divided highway, 4 lane, suburban, rural, narrow, mountainous, etc.