Basically it boils down to the shape of the muscle - and yes, there's nothing you can do about it, it's as individual as your face.
The long answer... It depends where the dent is.
Despite the neat anatomical drawings we get,
in fact the muscle may not be as even. Each muscle has an origin - where it anchors - and an insertion - where it pulls on. The pectoralis major has as its origin the sternum and the inside two-thirds of the clavicle (collarbone), and its insertion the humerus (upper arm bone). A tendon lies between the muscle and the upper arm bone. Your pecs may be quite full and long, or they may "peak".
It's probably clearer with your biceps. Flex your arm with palm up, notice how the biceps peak up, and don't go all the way to the inner elbow - you can feel where the muscle ends and the tendon begins. Some guys have got a long, full muscle, others have more of a "peak" to it. One guy's biceps muscle may end quite close to the inner elbow, another quite far. Those with more of a peak have got the muscle more concentrated in the middle with less near the tendon. This is just individual like your face, nothing you can do.
The same thing happens with your pecs. You can see this sort of thing in this guy,
just near his armpit there's a slight "dent" in. That's where his muscle is ending and tendon beginning.
Make sense?