The conversion factor is probably because the meter is probably reading volume but that's converted to energy at a standard rate, but the energy content is changing slightly, hence the correction. A therm is 10 000 BTU..
The Canadian Metric Practice Guide, December 2000, indicates that
1 therm = 100 000 Btu (Internatioal Table) = 105,506 MJ.
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Joseph B. Reid
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