Gas Volumes Follow Coefficients
The law of combining volumes states that reacting gases, and gaseous products, have volumes in simple whole-number ratios when measured at the same temperature and pressure. This law is also known as Gay-Lussac's law of combining gas volumes.
OpenStax Chemistry 2e explains that gas volume ratios in a reaction are given by the coefficients in the balanced equation when all gas volumes are measured at the same temperature and pressure in Stoichiometry of Gaseous Substances, Mixtures, and Reactions. Britannica also summarizes Gay-Lussac's finding that hydrogen and oxygen combine by volume in a ratio to form water in
Gay-Lussac's law of combining volumes.
The important word is gas. Coefficients can be read directly as volume ratios only for substances in the gas phase and only when those volumes are compared at the same temperature and pressure.