(Dressing myself with my Indexing Vicar collar here)
To amplify what Peter said, this is really the beauty of an index. The ability to provide keywords that represent concepts and not actual words. For example, perhaps you have a topic that describes a Sofa. Inside the topic proper, all the terminology used is simply "sofa". But some folks may call it a Couch. Some folks may call it a Divan. Some folks may call it a Love Seat. Others may call it a Sectional or Settee. So to effectively lead the user to the Sofa topic, you thoughtfully include all these terms and link them to the topic.
Another thing to note with regards to the index is that some folks have the expectation that clicking an index keyword will not only open the topic, but will also highlight the word inside the topic they view. That has never been a function of the index. It is a function of Search.
Helpful and Handy Links
Thanks Peter and Rick,
You really don't need to tell me about the beauty of an index; for years now I have been sweating to make good indexes (indices?)
But my dear departed colleague (irony) has left us with an inconsistent index and we want to find a way of cleaning it with a minimum of effort.
He created the index with RoboHelp's index wizard; as you know that lists topics containing the keywords and the author only has to decide whether the mention is merely incidental or justifies an index entry. In many cases he was too lenient and this results in an index which is really no better than a search.On top of that he edited and in some cases split topics AFTER indexing.
What we would like is a tool which leads us to topics where the index entry is NOT in the topic; then we can check these individually.