Why do different beans grind differently at the same setting?

The same grinder setting produces genuinely different grinds from different beans.

Density

Light roasts are denser. Less of the bean’s structure has broken down, so it resists the burrs more and fractures differently.

Dark roasts are brittle and porous. They shatter easily, producing more fines at the same setting.

What this means practically

Switching from a dark to a light roast usually requires grinding finer — the light roast produces a coarser effective grind at the same gap, and it is harder to extract anyway.

Moisture content

Fresh coffee holds more moisture. As it ages it dries slightly, and it grinds marginally finer.

That is part of why a bag needs re-dialling over its life.

Ambient humidity

Coffee is hygroscopic. On a damp day it absorbs moisture, grinds slightly differently, and extracts more slowly.

This is a real effect and it explains shots that run long for no other reason.

Processing

Naturals are often slightly softer than washed coffees of the same roast level, and can grind marginally finer.

Decaf is a special case: decaffeination makes the bean more porous and brittle, so it grinds noticeably finer and extracts faster. Start a step coarser than usual.

The consequence

Expect to re-dial for every new bag. Not a failure of your grinder or your technique — just the coffee being different.

Two or three shots is normally enough.