Ingrown hairs cluster on the neck far more than on the cheeks, and the pattern is consistent enough to point at a structural cause rather than at technique alone.
Neck hair grows in a different direction
Facial hair on the cheeks and jaw generally emerges at a fairly steep angle in a broadly consistent direction, which makes it straightforward to shave against or with.
On the neck the growth pattern is far less orderly, with hairs emerging at shallow angles and frequently changing direction across small areas, including swirls and opposing patches.
A single stroke across that area therefore shaves some hairs with the grain and others against it, which produces different results within a few centimetres.
A close shave cuts below the surface
Skin is pulled slightly by a blade passing over it, and when it springs back the cut end of the hair can sit below the level of the surrounding surface.
The cut also leaves a sharp, angled tip rather than the tapered end a hair grows naturally, which turns the emerging hair into something closer to a needle.
If that sharpened tip meets the follicle wall on its way out, it can penetrate sideways into the surrounding tissue instead of emerging through the opening.
Curl makes the problem worse
Hair with a curved cross-section grows in an arc rather than straight, so a tip starting below the surface is far more likely to curve back into the skin.
This is why coarser, curlier beard hair is associated with a much higher rate of ingrown hairs and with the persistent bumps that follow repeated shaving.
The neck compounds it, because shallow growth angles mean an emerging hair travels a longer distance through tissue before it reaches the surface.
What the body does in response
A hair growing into tissue is treated as a foreign object, and the immune system responds with inflammation, which is the red raised bump that becomes visible.
Repeated episodes in the same area can leave darkened marks and firm scarring, since each cycle of inflammation affects the surrounding tissue.
Picking at the bump adds mechanical damage and a route for bacteria, which is why the advice is consistently to leave them alone.
What changes the outcome
Shaving with the grain and accepting a slightly less close result reduces how often the cut end finishes below the surface, which addresses the mechanism directly.
Fewer blade passes over the same patch and a sharp blade both help, since repeated stroking removes surface cells and irritates the follicle opening.
Persistent, widespread or scarring reactions are a dermatological matter rather than a technique problem, and are worth assessing properly rather than working around indefinitely.