They're all related, but as CNG is primarily Methane, which, from the diagram, is obviously lighter, it requires greater storage pressures to achieve proper energy density in storage, unlike Butane/Propane, which can be stored in quantity in lower-pressure vessels.
That's why CNG in vehicles is not as popular as LPG, and it's mostly buses that use it, because they can take the extra weight of the high pressure tanks.
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