Gas water heater burn rate

I recently installed a replacement gas water heater. Just for fun I calculated the rate at which it should consume gas given its 40,000BTU rating. The numbers said it should consume one cubic foot of gas every 97 seconds (assuming 1075 BTU per cubic foot).
So I went out and timed the dials on the meter and discovered it took 130 seconds to consume one cubic foot. That rate suggests the heater is running at 29,770 BTU. About 25% below the rating.
My first thought is something is wrong with the heater. Any other thoughts would be appreciated.
Note I have used this method on two heating furnances. One at this house. Both furnaces use gas within a few percent of the calculated rate.
Thanks
Derek Toeppen
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