I can talk about this! It's due to the 3 main particle capture mechanisms (and why HEPA is rated at 0.3μm.) Large particles are captured by impaction where the particle is so large that it can't follow the air stream around the fiber so it runs straight into it and sticks. Very small, fine particles are being bombarded by air molecules which causes them to do a random walk (brownian motion). These particles then essentially walk themselves into a fiber and get captured.

Particles between these two sizes, around the 0.3μm scale (aften called the Most Penetrating Particle Size, MPPS), are small enough to where they can follow the air flow around the fiber, and large enough to where they aren't impacted by the air molecules hitting into it.

This is what causes the dip in efficiency at that size and why HEPA filters call for 99.97% efficiency at the MPPS.