Short answer: because it has a lower carbon:hydrogen ratio.
NG is mostly methane (minimum 88% if I recall the stickers on the PG&E "pumps"), with 1 carbon and 4 hydrogens. The rest is made up primarily of also fairly short (2-4 carbon) chain hydrocarbons.
Traditional gasoline is heptane (7 carbon) and octane (8 carbon).
Methane's ratio is 0.25.
Heptane's ratio is 0.4375, and octane's is 0.4444. (Longer chains asymptotically approach 0.5).
I get the same economy (30-33 mpgge) in my Civic GX as in my gasoline Civic.
However, let's compare carbons based on molar mass (16.042 g/mol for methane and 100.21 g/mol for heptane and octane is 114.23g/mol). That's 160 moles of methane per GGE and 27.5 moles of heptane per gallon. That's a ratio of 160:192 of carbons. For octane, that's 24 moles per gallon, about the same ratio. It's not 1:4, granted, but these are just hand-wavey approximations for the gasoline, which has the added complication of ethanol. I'm also assuming complete burning, which is patently false, considering that, before modern emissions controls, it was possible to commit suicide by carbon monoxide poisoning from engine exhaust.
NG is mostly methane (minimum 88% if I recall the stickers on the PG&E "pumps"), with 1 carbon and 4 hydrogens. The rest is made up primarily of also fairly short (2-4 carbon) chain hydrocarbons.
Traditional gasoline is heptane (7 carbon) and octane (8 carbon).
Methane's ratio is 0.25.
Heptane's ratio is 0.4375, and octane's is 0.4444. (Longer chains asymptotically approach 0.5).