It is quite easy to explain. When you are in a high NW category, you tend to lead a "more expensive" lifestyle and thus your expenses will be higher.

A family living in a HDB flat and who does not own a car will have a far lower household expense than one living in an expensive condo and drives a fancy car. The HDB family is likely to be flying budget airline when on holiday while the high NW guy will fly on full service airlines or even business class. Many of these people would want to maintain this lifestyle. They will strive to build their wealth to generate the passive income they need and complain if they cannot meet it.