Why does my grinder retain coffee?
Retention is grounds stuck in the burr chamber and exit path, held there by static and by dead space in the grinder's design. Use RDT, a few drops of water on the beans before grinding, plus a bellows puff, and most grinders drop to well under a gram.
Every grinder holds back some coffee. Two things cause it. The first is geometry: hopper-fed grinders were designed to always have coffee in them, so their chutes have corners, ledges and long horizontal runs where grounds simply sit. The second is static, generated as beans are sheared, which makes fine particles cling to plastic chutes and metal walls instead of falling out.
The reason it matters for single dosing is dose accuracy and staleness. If you weigh 18 g in and only 17.2 g comes out, you are not pulling the recipe you think you are, and next time some of yesterday's grounds join today's shot. Below about half a gram this is a non-issue. Above a gram it starts corrupting both dose and flavor.
The most effective fix costs nothing. RDT, the Ross Droplet Technique, means adding one or two drops of water to your beans and stirring before grinding. That tiny amount of moisture dissipates the static charge. Clumping collapses, the grounds fall out of the chute instead of sticking, and the mess on your counter largely disappears. Use a spray bottle or a wet spoon handle, and use very little: you are dampening the bean surface, not soaking it.
Combine it with mechanical help. A bellows on the hopper lets you puff air through the burr chamber after grinding and push the remainder out. Tapping the grinder body firmly a few times does much the same thing. Some people also do a bean dump and a single-bean purge, but with RDT and a bellows that is usually unnecessary.
If your grinder still retains several grams after all that, it is a design limit rather than a user error. Grinders built around a hopper and a doser were never intended for single dosing, and the aftermarket funnels and chute mods that exist for popular models can help but rarely eliminate it. That is a legitimate reason to consider a purpose-built single doser, though be honest that you are buying workflow rather than a better cup.