Buildup ? You mean the normal amount of carbon blow by which 90% of cases show and suffer from ?
The non snarky version is:
There is clearance between the neck and the chamber. When the powder does its explodey thing and pushes the bullet out, there is a minimal amount of blow-by around the neck before the explodey does full pressure and "fire forms" the case into the chamber completely. And when it does that, it presses the brass to the chamber wall, it stamps the carbon onto the brass neck, which you are seeing. "Ink blot" sort of effect.
Over sized brass (if you resize too much, like 8thou shoulder bump) will result in the carbon moving around the neck and onto the shoulder before the full explodey, resulting in carbon all the way down the shoulder and even potentially a mm or 2 (like 40thou) onto thr body. Rare, but ive seen it.
