This is based on a library's subscription to OCLC services. Libraries need a certain subscription level in order for their holdings to be visible outside of a subscribing library's IP range. This is why you see different results when you are searching WorldCat.org at your library. You are within a library's IP range and can see all the libraries that hold the item (regardless of subscription level). When a user is outside of your library's IP ranges they can only see libraries that have a subscription that makes their holdings visible in WorldCat.org.