There is a simple and a general explanation to it. As it gets higher up from the sea level, the elevation gets closer to the lowest point simply because the distance from the highest point keeps decreasing as you get closer. The elevation is the distance left between the highest point and the current point.
Keep doing each step manually, and see where it slips up:
−7.9+8.2−3.4+2.1=−1
−7.9+8.2+(−3.4)+2.1=−1
−7.9+(−3.4)+8.2+2.1=−1
(−7.9+(−3.4))+(8.2+2.1)=−1
−11.3+10.3=−1
