You are 20ft away from the hole. The ball travels in its line up 4 inches, then down 3 inches, then levels off, then falls another inch a foot from the hole. The game will say the ball to the hole is 0 inches in height comparison. By tracing the triangle over the terrain you will see the hills and valleys, and perhaps that 20ft putt, acts more like 18ft, or 22ft.
All the game tells you that the ball and hole are at the same height, but doesnt explain whats in between. Pay attention to the grid and try moving the cursor along the line you want. The correlation between up hill putt power increase and downhill putt power decrease is not the same.
Also works well for chips and putts, i.e at andrews rd hole fescue near the green, chip reads level to hole when you know that its going all downhill from landing zone.