Do you mean behavioral traits or physical traits? Physically, you would expect them to be very similar, but behavior is shaped by environment as well as genetics. And even if the pups grow up in the same environment, they have different experiences. One may get more/different attention from the dam or more/different attention from siblings. If raised with kids, one might be the favorite of one child while the other is the favorite of another child so they're both spending time with different people who handle them differently. So many factors that can have an influence on their behavior.