The tool is calibrated to read the porosity of a water-filled limestone, or, on another scale, a sandstone. The scales are not interchangeable. A sandstone reading presented on a limestone scale is the wrong porosity until it is converted.
Gas has less hydrogen than water, so a gas zone reads low. Shale reads high because of bound water. Plotted next to a density porosity, gas shows up as crossover: the neutron low, the density light. The curve is a hydrogen index turned into porosity units, not a direct count of the pore space.