How did First Utility turn a £950 refund into £1,562 debt?

It admitted it calculated our bill wrongly and paid us the money – then it asked for more back

A gross error by First Utility will leave me and my young family in financial difficulties. When we sold our house last summer I called the company with the final meter readings and was informed that we were due a £950 refund, which they paid.

Six months later a letter arrived at our new address asking me to settle an outstanding bill of £1,562. First Utility told me they had made a series of billing errors and instead of the £950 refund I actually owed them £1,562.

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Source: the guardian
How did First Utility turn a £950 refund into £1,562 debt?