Why can someone eat poorly and not have gut issues, but then someone else can eat so healthy and struggle a lot? What is the main factor?

Why can someone eat poorly and not have gut issues, but then someone else can eat so healthy and struggle a lot? What is the main factor?

Both people have good bloodwork.

Is it mainly four things, bile, motility, bacteria, and stomach acid? Or is it more about genetics? What is the main driver?