Why can using too little sunscreen make the label SPF unreliable?

Answer: The film gets too thin

The film gets too thin ✓ — Right: sunscreen is tested as a film at a standardized thickness, 2 mg per square centimeter. When you use much less, the film becomes thinner and often less even, so the test condition on the label no longer matches your skin. That makes the printed SPF less reliable as a prediction of real protection. The surprise is that the number assumes a physical layer, not just a product name.

Skin absorbs the SPF — This points to the wrong level of the problem. The label SPF is earned with a measured film thickness, so the first question is whether enough product is sitting on the surface in a continuous layer. If you under-apply, the setup no longer matches the test. The practical failure is geometry: coverage and thickness, not the skin secretly subtracting the number.

The SPF halves neatly — This is a neat arithmetic guess, but the label does not simply update itself to match your thinner layer. SPF is measured under a standard application thickness, so changing the film changes the test condition itself. Some areas may be covered while others are sparse. The label remains true for the lab setup, but your skin may not be wearing that setup.