I have a mission plan where the copter takes off and goes to 20m. The altitude for the 3rd waypoint is 110m, so while moving from 2nd to 3rd waypoint the copter rises to about ~123m and then reduces to 110m while moving to the 4th wp.
Randy is correct. With spline waypoints if you want to estimate what will happen at that waypoint (#3) take the waypoint before it (#2) and the waypoint after it (#4), draw vectors in 3D space from #2 to #3 and from #3 to #4. The actual path of the aircraft as it passes thru the waypoint (#3) will be approximately the average of those two vectors.