Without viewing the elevations that were specified within the editor, I can only guess that a value, or cluster was set at the top of the hill and one at the bottom. If the designer does not designate specific elevations on each tile in that area, this leaves the area in-between to populate its own slope. I've seen this when I've created maps and had to work around it by handpicking specific elevations along the way to override what the editor would have filled in. It takes time to do this, trial and error, but you get the end result you want.
I probably didn't tell you anything you didn't already know. Yes it would be an improvement to have a consistent slope, I don't know how it is designed, coprocessor dependent . Spent a lot of time camera surfing terrain and exiting back to editor, repeat and repeat.
Also, any elevation changes made anywhere on the map can cause a ripple effect, because now the editor has to recalculate even the most minute slope changes for all open pathed tiles. I'm assuming that is not the case here because you made no changes.