Not all indexes are the same.  This is especially true for those taken of newspapers.  The number of newspaper indexes released has greatly improved with the increase of volunteer projects.

With the size of newspapers, it is not possible for an indexer to capture all the information.  Not all indexing projects include all articles.  Some are just for social pages and obituaries.

What is nice is the ability to browse an entire newspaper online and look for relevant newspapers when a name is found in the index.

James Hansen’s chapter on Newspapers in The Source goes into details what can be found in newspapers.