if you mouse over the purple test color in your preview window and check right under the previe window where it says Y Z slope, you notice that the Y is roughly 2.61m so its correct, try always to check the altitude with ur mouse pointer, there is nothing wrong in your example.
to fix it, instead of using "limit maximum altitude" use "llimit minimum altitude" and set it to 3.2, provided you want that area exluded by the shader, you can use both in combination to select a particular "stripe" or zone like ulco said
