Probably has something to do with the LOD (Level Of Detail) built into the mod. Most mods have less detail the farther out the view is to save on frame rates. What that amounts to is that there are a number (which can vary by mod makers) of different detail levels so that as you approach, or go away, the detail amounts change in-game so as to not cause too great of a hit to the video card. If there aren't enough transition LODs, and because you have yours set to 'high', may be the reason you're seeing the changes.
At least that's my understanding of LOds and how they affect the game.