Discrepancy between zappi and octopus power usage

[DelBoy]

I have been monitoring the zappi kwh usage when charging and then octopus began stating how much they stated how many kwh's were being used and I found that the octopus amount was 10% less that the zappi stated. Just an observation but if you are using the zappi figures to monitor usage if may be wrong. Anyone one else noted this?

 

[Kenp1]

The Hypervolt is the same. I think the Octopus app is giving how much it thinks the battery has charged which will be less than the electricity used, this seems to be wrong because if ask for 25% added to the battery I will probably get about 27%.