Indexing is both an art and a science. Good indexing balances the rules that need to be followed, such as style guides and grammar, with subjective decision-making in term selection and arrangement. The concept of indexing consistency deals with the creative aspects of the indexing process, and it is a key component in understanding the construction of indexes.
Indexing consistency can refer to either interindexer consistency (a comparison of the work of two or more different indexers) or intraindexer consistency (a comparison of the work of the same indexer at different times). The subjective nature of choosing index terms will inevitably result in discrepancies. Indexing consistency can be considered a subset of editorial consistency.
For an in-depth look at interindexer consistency with side-by-side examples of indexes created by two different veteran indexers, check out Inside Indexing: The Decision-Making Process by Sherry Smith and Kari Kells.