An adverbial phrase meaning 'contrary to expectations' or 'against what was expected.'
予想に反して、試験はとても簡単だった。
Contrary to expectations, the exam was very easy.
天気予報は雨だったが、予想に反して晴れた。
The forecast said rain, but contrary to expectations it was sunny.
彼は弱そうに見えたが、試合では予想に反して強かった。
He looked weak, but contrary to expectations he was strong in the match.