The app inacuracy

I’ve used the app over the past to send my electricity readings through and as an indicator of the balance on my account. When I thought of checking an actual bill after last winter I was shocked to realise that I underpaid the account by £450ish. I’d never would’ve left it run to such a debt, shouldn’t it be for the app showing a different account balance.
Currently app says I’m £89 in debit but actual statement for October is £239!!

Is anyone having the same experience? And is aware of it?

Many thanks MH