Diffusion takes care of the smallest particles. These small particles do not strictly follow the stream lines. They follow the brownian motion and move randomnly due to collisions with gas molecules. Because of this movement they are likely to collide with the fibres. The smaller the particle is the higher its freedom of movement and therefore the higher their probability to encounter a fibre. Captured by diffusion becomes more important with decreasing particle size.
The total filter efficiency is the result of the combination of these three filtration mechanisms.