Answer: Because some of the processes are also known as incomplete combustion.
Step-by-step explanation:
Combustion reaction is defined as the chemical reaction in which a hydrocarbon reacts with oxygen gas to produce carbon dioxide gas and water molecule. The chemical equation for this reaction follows:
![\text{hydrocarbon}+O_2\rightarrow CO_2+H_2O](https://img.qammunity.org/2020/formulas/chemistry/high-school/9j7rvgqpgy0k8j0r5dixkloadwib1ixmit.png)
If supply of oxygen gas is limited, it is known as incomplete combustion and carbon monoxide gas is also produced as a product. The chemical equation for this reaction follows:
![\text{hydrocarbon}+O_2\text{(limited)}\rightarrow CO+CO_2+H_2O](https://img.qammunity.org/2020/formulas/chemistry/high-school/8e7t2lpy6jooenxmsq5bkuc9hf914wvala.png)
Thus, we cannot say that all the combustion reactions produce carbon dioxide and water as products because it depends on the supply of oxygen gas.