Here's why:

* A horizon: This is the topsoil, the layer most directly affected by biological activity. It's rich in organic matter, microorganisms, and nutrients. It's where plants grow and roots penetrate, leading to constant decomposition and mixing.

* B horizon: This is the subsoil, less affected by biological activity than the A horizon. It's often richer in clay and minerals than the A horizon and may have a different color due to the accumulation of iron and aluminum oxides.

* C horizon: This is the parent material, the bedrock or sediment from which the soil develops. It's usually less weathered and has less biological activity than the other horizons.

While the A horizon is usually the thickest, it's important to note that the thickness of each horizon can vary significantly depending on the soil type, climate, and other factors.