Possible cause for Google Maps / bicycle odometer divergence (tire size setting correct)
Google maps probably treats roads as not having any width, and corners as point turns. Unlike bends, if you zoom in you tend to see a sharp 90 degree change in line on a curve This will cost you the length of the arc you really take. 

It certainly omits all the lane changes, obstacle avoidance and similar manoeuvres. 

However there is some variation in tyres - particularly some of the anti-puncture tyres have a bit of extra thickness; inflation vs. weight on the wheel will also have an effect. So you're better off seeing the wheel circumference based on counting the wheel revolutions over an accurately-measured straight line.