What does that mean? (Assistant Match Report)

[Tactician]

I had a tough match where I put 5 defenders, 8 men behind the ball (later changed to 9 men behind ball) and played defensive.
Final Result: 1-0
The opposing team gets a red card in the first half itself, and later I went attacking and even all out to try get goals but in vain. His goalkeeper got a 9.6 rating.

Assistant Report
We totally underestimated how good they were.

Wt*, how did I underestimated how good they were? If that was the case, I would have played openely without any care instead of putting 8 or 9 men behind the ball. What does that mean 'We totally underestimated how good they were'?