Sorry Grumpy, but you are just plain wrong.
A BTU is a British Thermal Unit and as you explained it is the amount of heat necessary to raise the temperature of a pound of water one degree Fahrenheit.
Natural gas is measured by the cubic foot but is sold by the therm. A therm is equal to 100,000 BTUs as you also stated.
HOWEVER, natural gas is a mixture of many gases, predominantly methane. Because it is a mixture the heating value varies according to the mixture. Because of this there is no guarantee that exactly one cubic foot of gas will yield exactly 1,000 BTUs. Since the gas is sold NOT by cubic feet but by heating value (in therms) it is necessary for the gas supplier to apply a correction factor to the metered (cubic feet) gas flow to adjust for the actual heating value of the metered gas. One cubic foot measured may have a heating value as low as 900 BTUs or as high as 1,100 BTUs. It AVERAGES about 1,000 BTUs but it is NOT an absolute. By applying the correction factor the customer is paying not for the metered VOLUME but the ACTUAL heating value.
I think my 35+ years as a degreed energy systems engineer beats whatever your time as a plumber, machinist and whatever else you have done.