In web maps, there are two coordinate: wgs84 geographic coordinate and web mercator project coordinate. The displayed scale of a world map is the value at equator, and one 256*256 pic can contain the map in web mercator level 0.
So we can calculate the scale like this, 1:(2*pi*6378137)/(256/96*0.0254)=1:591658710.909131. But in the similar Esri provided service is 1:591657527.591555(Esri Ocean Map).
So, what's the affect in map display with the tiny difference? And how Esri calculate his scale?
ps: I was also confused with the same problem in wgs84.