Some thoughts. If your boiler is really old, there is a chance that a lot of heat goes up the chimney before that boiler jacket is heated up and the circulator starts moving water. Come to think of it, a lot would be going up the chimney afterwards too. Any boiler manufactured in the last 30 or 40 years should be in the vicinity of 75 to 80% efficiency.

Another thing is insulation. My house is a 100 years old, balloon construction, tongue and groove siding, horse hair plaster over lath, and it had ZERO insulation. I hired a company to come and foam insulate and the savings were incredible. Cost me a bit less than $6000 Cut my heating and cooling in half, no exaggeration. A brick house they'd have to come inside and drill through the plaster to inject

One other thing might be how the gas company bills. Sometimes they estimate and then update after a real reading.