Rich gas and lean gas are the two types of natural gas that are distributed in Belgium via different networks. The difference between them is their calorific value, i.e. a different quantity of energy. In other words, 1 m³ of lean gas provides less energy than 1 m³ of rich gas.
When using a wood-burning stove, no-one would want to pay the same price for a cubic metre of pinewood as for a cubic metre of oak. The volume may be the same, but pine wood contains far less bulk (and hence less energy) than oak.
The same is true of gas. The two sorts of gas available in Belgium have a different calorific value, i.e. a different quantity of energy:
How come? As the name indicates, natural gas is a natural, not an industrial product. Its composition varies in time and depending on the country of origin. This is why the gas system operators recalculate the calorific value every month.
In other words, less energy is released by burning 1 m³ of lean gas than by burning 1 m³ of rich gas. For instance, heating the water of the same bath will use more lean gas than rich gas.
Luckily, that does not mean that you have to pay more, because your bill is calculated on the basis of the quantity of energy (number of kWh) and not on the basis of the quantity of gas (number of m³) that you have used.
Whatever type of gas you receive, everyone actually pays for the energy that they use!