I will explain this in story form. I have a Bushnell 1500 Model Range Finder.
Some have various modes you can choose from. Mine has one setting for Trees and Brush, one which has a Target on it and another for regular op. etc. Since the reflective material of Trees and the like is not as reflective as that of a car, house, rock buff or other very solid object. The return reflection coming back from the brush or tree is not as strong a reflection. Therefore even though the unit boasts of 1500 yards it would not pick up a reflection of a tree 1500 yards away. Maybe even half as much with those objects. Also even if it was a huge tree with lets say of 20ft. diameter around the base. It still has bark that is a softer absorbing material. If it was a 20ft. diameter power pole of smooth surface it might very well work at 1500 yards due to the high solid reflective properties (or surface on the pole)
So that is why the units have the various settings. If I do not use the correct setting I could very well get a false reading or not get a reading at all due to the improper selection of the signal the unit was emitting. Just remember the softer the item is, as well as size, makes a difference in signal bouncing back to the unit.
Hope this helps explain how they work.