Its shows us that genes are important for many things, but they do not completely dictate everything about us. Our personalities do involve genes, but are also very dependent on our upbrining and past experiences (what we call “environment”). Because no two people have identical experiences, not even identical twins, they will not have identical personalities.
This combination of genes + environment is important for many of our characteristics, not just personality. The reason some people get lung cancer is because of combination of cancer genes and smoking. Its this combination that makes it very difficult for scientists to understand how things work.