Generals like Haig the Incompetent, had very little reliable information what was happening in the major offences on the German side. So when the massive French and British losses piled up, they assumed that this meant that German losses were as bad or worse. (They weren't)

And the consequence of that at the middle army management level was bragging rights about how many of your men you'd managed to get killed. Source: Hochschild, To End all Wars.