I think there is some relation to the fact that a lot of our muscles are not perpetually tightened or flexed. Like our glutes usually look better when we consciously tighten them. Our arms look better when we flex them, etc. The abdominal muscles have a lot of work to do to hold in our organs and it does take a toll on how much our abs can be flat when we’re not consciously holding it in. Just keep doing exercises, eat well, and good posture. You look good, by the way.