No you don't but you also don't know what people do day to day, so you're entirely reliant on people telling you.
You can't measure someone else by your own usage especially when you use hardly anything. I use 16 kWh per day normally vs your 4, with or without solar.
The only proof is in the pudding, which is in the Octopus grid stats, which removes all local adjustments and things like inverter stats skewing figures.
I don't even need exact figures for me to see that someone like
@john_smith is a very heavy electric user. The stats they posted a few days ago showed they used 90% of all the solar they generated for the whole year, this is a very high efficiency figure and is only possible if you use a lot, especially when this figure encompasses things like generating 600 kWh in the best month.