An uneven floor makes every careful measurement answer to the actual room.

Plans assume conditions that real rooms do not have. The drawing is square, the floor is not, and every careful figure has to meet the thing itself before it means anything. This is not a failure of planning; it is what planning is for, to be corrected on contact. The room decides, and the paper only proposes.

Before committing to anything measured, test it in place. Hold the shelf up, walk the route at the actual hour, run the process with one real case. The mismatch you find now is far cheaper than the one you find after cutting.