How are service areas created in ArcGIS Network Analyst?

I want to create a service area map of Burma in order to spot the areas that are within xx kilometers from a set of cities.

The problem is that the generated polygons aren't exactly what I expected.

In order to understand why, I tried to generate service areas on a very simple network where the greens dots are my three facilities :

The first network is a simple triangle. We can see that computing service areas on this shape completely failed.

However, if I complexify a little bit the network by adding edges inside the triangle, it runs smoothly :

In front of these results, I wonder if someone can explain to me how a service area is generated. More precisely, how does ArcMap interpolate the values of the polygons in the areas where there are no streets?