I’d check they are converting the unit of measure (m3 or ft3) into kWh’s correctly. Also, check how many digits of the meter dial they were billing prior to the change against how many they are now.
You can do a sanity check yourself by using the formula below. You say it’s ft3 so that’s the formula provided below. for m3 just drop the 2.83 multiplication in step 1. Note the CV and the temp and pressure figure will vary slightly so it may be different on your bill.
For ft3 meters – multiply units used by 2.83.
Multiply by the temperature and pressure figure (1.02264).
Multiply by calorific value (39.25)
Divide by 3.6 to get the number of kilowatt hours (kWh)