the strange thing, is butt muscles don't look like that
Uh.... are we looking at the same thing here?
You don't see that?
Here's a hint: genetics and muscle destruction/repair cycles aren't perfect.
Go look up "six pack" and "abs" and you'll see about a billion variations of muscle shape and definition of the rectus abdominus.
There's the "perfect" muscle as defined by scientific drawings, and there's reality.
There's a reason things like fingerprints are different for everyone, and it's not just because the human body wants to cooperate with law enforcement and provide strong evidence.
Every heart is quite different when you get into the details of each and every section. One quadrant might be larger, the outer layers of muscle not as beautifully striated as another's, etc.
And that's just nature - now bring orchestrated, conscious movement into the mix, and you produce a billion variables. If you do a certain exercise just ever so slightly different than someone else, and do that movement exactly the same every time you work that muscle group, you will likely produce a different muscular growth pattern than that other person.