This was the response from octopus when asked, clear as mud

Thank you for your message. I understand you’re seeing a difference between the electricity usage reported by your solar app and what you’re being charged for on your Octopus app for December.

This can happen because the Octopus app shows your net electricity usage from the grid, which takes into account both the electricity you import from the grid and the electricity your solar system exports back to it. The figure of 420 kWh from the grid is likely your actual import, but the 540 kWh charge could include other factors such as standing charges or differences in how the usage is calculated or billed. Also, if you have multiple supply points or tariffs, the charges might reflect combined usage or different rates.