The Bishop and Knight and are of similar value in the game of chess. As their movements are quite different, you could say their fighting styles are quite different. So in different situations, games of a different nature determine which is stronger.
The Knight is a short range piece whereas the Bishop is a long range piece. Therefore Knights tend to be stronger than Bishops in closed situations. Bishops on the other hand are stronger than Knights in open games.
Knights prefer when all pawns are on the same wing. This negates the Bishop's advantage over him. The Bishop does much better when there are pawns on both wings.