Well, we all know that Organization XIII was under Xemnas's orders, right? Well, let me explain this:
Organization XIII was created by Xemnas to form an obstacle in Sora's path so that Sora would killed more Heartless for KH and Xemnas would have more time to build KH.
In other words:
Xemnas created the org. to distract Sora while he creates KH. Meaning that he promised the other members their full existence and he knew they wouldn't stand a chance so he used them to et them killed to have KH for his own.