The User's Manual describes what happens in each of the Eco modes.
Keep in mind that all range estimates are based on your previous driving. If you set a destination into the nav system, it will use that information to make an assessment of how far you'll be able to go. Keep in mind, otherwise, the car has no idea where you're going, the types of roads, the speed limits, or the terrain, so it uses the history data. If the original estimate was more economical than running long-distance down a highway at higher speeds, then yes, it won't be right. Also, at a steady speed, you're not accelerating or in regen much, so most of the energy use is drag and friction, again, not all that much an Eco mode can do. It's more likely to make a difference around the city in stop and go where you're starting/stopping much more frequently. Eco Pro makes it harder to go faster, but you still can. Drag goes up rapidly with speed.
Probably the biggest thing is that the computer is looking at conditions at least a few times a second...how often are you? Some are better at that than others.
2011 535i x-drive GT, 2014 i3 BEV