Thinking about what they actually said, they are trying to say that because the gradient of the ground is so steep that even though we are within 30cm at the highest point, the context of the land (the gradient) means that the reference point should not be the highest point. I think they are enacting the bold letters below which basically caveats the rest of the text.
2 Interpretation - (2) Unless the context otherwise requires, any reference in this Order to the height of a building or of plant or machinery is to be construed as a reference to its height when measured from ground level; and for the purposes of this paragraph “ground level” means the level of the surface of the ground immediately adjacent to the building or plant or machinery in question or, where the level of the surface of the ground on which it is situated or is to be situated is not uniform, the level of the highest part of the surface of the ground adjacent to it.
If the gradient was less then I think what people have been stating is correct but in our case I think they are asking for planning.