Because this nucleophile can attack the carbon either from the top side or from the bottom side. When it attacks from the top side, it forms one of these. When it talks from the bottom side, it forms the other isomer. Now, what is the difference between these two animals, these two isomers? Well, if we examine the stereochemistry of carbon number one, we'll see that the stereochemistry of carbon number one is different in these two cases. So all the other atoms are arranged in the same exact way.