Lawns and vegetation have a limited ability to absorb water based on site specific conditions. Once saturated or frozen, water can no longer be absorbed into landscaped areas. Often, only a limited amount of impervious surface areas can be directed to an impervious area before its ability to absorb the additional water is diminished.
Buildings, garages, sheds, houses, etc. are hard surfaces that directly shed stormwater off their roofs. These surfaces eliminate large impervious areas where water can infiltrate.
Pavements like concrete and asphalt are designed to drain stormwater to a low point to prevent ponding on the surface. Water does not infiltrate standard pavements.
Standard brick pavers are typically installed over compacted base material, and the joints in between the pavers are also filled in with compacted material. These act similar to pavements, and typically do not allow any stormwater to infiltrate.
Gravel materials used for parking, roads, driveways and walkways is tightly compacted, and does not allow for infiltration. Water will always find the path of least resistance, and will tend to run downhill over compacted gravel rather than infiltrate into the underlying soils.
While stormwater may be contained within pools during a rain storm, the pool itself does not allow water from other parts of the property to soak into the soil. When the pool is drained, the stormwater that was collected will be sent to the sewer system. Considering pools as impervious surfaces is consistent with many other stormwater utilities throughout the country.