Given that the Italians pretty much invented modern accounting during the renaissance...
Anyway, could be due to different rules of accounting. If so, not so much illegal practices by the subsidiary, but a big blunder by the parent company. (It is BT after all.)
Example: when Mercedes Benz got ready to morph into DaimlerChrysler and to get listed at the NY stock exchange, they published two financial statements in parallel for that year; one drafted as per the German rules of valuation, one drafted as per the US rules. Version one: 2 Bn DEM profit. Version two: 0.5 Bn USD loss. No, accounting is not like engineering.
"Next week on show and tell: California divorce law - how to hide your assets." -- (Christopher) Titus