- WorldCat Discovery
It is likely that these databases are part of OCLC's central index, which means that the providers have given OCLC metadata about the content. While providers are required to give us certain metadata, they often do not provide all of the metadata that is available in their platforms, which is part of why you are seeing a difference in search results in WorldCat Discovery versus the database platform.
There is also a de-duplication process that is applied when multiple databases are searched so that if an item is found in one database, it is only displayed once so users don't have to scroll through multiple results for the same item. Usually, if WorldCat.org is included in a search, and the item is found there, this is the database which displays the item.
For more information see Available Collections in WorldCat Discovery.