I had a very copacetic bike trail/bike lane commute in Colorado with only one traffic light forcing me to stop for any length of time. My ride home was always about 5min faster than the ride to; probably due to favorable net elevation change and having more vigour, in the afternoon. Anyway, except in adverse conditions, my homeward ride was typically 35-40 min. gross elapsed time on a 13 mile distance. There were 3 significant hills along the way, and though it seems counter-intuitive, I think now they made me faster overall due to the long-term strengthening effect. The reason I say that is now I have a basically flat bike trail commute in MI, again with only one significant traffic light, and a distance of 11.5 mi, and I think my fastest elapsed time so far is around 39 min, with 40-42 being more typical. To be fair, there is one area of bridge construction where I have to slow way down, but I doubt that is even costing a full minute.
My conclusion is either that the distances reported by GoogleMaps are not accurate, or more likely that I have not built up to the condition that I was in in CO, in my current MI commute.