The longer answer
The comb is the single most important fit dimension on a shotgun, because it places your eye — and a shotgun is aimed with the eye, not with a sight. If the comb is too low, your eye sits below the rib and the gun shoots low. If the comb is too high, your eye floats above the rib and the gun shoots high. A gun that fits puts the dominant eye looking straight down the centre of the rib the instant the cheek lands.
Many target guns ship with an adjustable comb — a small mechanism that lets the shooter raise, lower, and sometimes cant the comb to dial in fit. Fixed-comb guns can be shimmed or restocked by a gunsmith. Either way, comb fit is worth more than a more expensive shell.