Anybody have any idea why it takes longer to fly from one city to another in atmosphere than it does at ground level?

An example in a yt1210 it takes me 47 mins to move one square to prospect again, but at ground level its 13mins to get across my current city and then 20 mins on cross terrain (a saving of 14 minutes). Now i'm not complaining about the time but I am curious as to why the difference.

I think it would be simple geometry really. Since you are travelling on the planet surface, you're going dirtectly along an arc. but if you draw a line at a 90 degree angle from the ground to space the two points on the arc are actually further apart as you go further away from the surface of the planet. If the world was flat it would take the same time, but since you're dealing with an arc the two lines get further away from each other as you get further from the points or origination.