It appears the fractional "Used kWh" amount is included by TeslaFi in multiplying by cost per kWh, but Tesla only charges by the whole kWh, meaning a rounding function might be needed.
Real life example:
Supercharger cost (overridden in the properties of the location): $.23 / kWh
Used kWh: 36.2
TeslaFi calculated cost: $8.33
Actual bill from Tesla: 36 kWh @ $.23 = $8.28
Further investigation shows they round to the nearest whole kWh amount.
Customer support service by UserEcho