Indexes to print materials and printed books that are themselves indexes are not all created the same. Always read the preface to see exactly what material was used to create the index. For a book that has it’s own index, read the introduction to the index (if there is one) too see if there are any comments that are relevant.
And in books that are themselves indexes, determine how the index is sorted. It might not be strictly alphabetical–I used an index where the items were sorted by year and then by name. I’ve used indexes where the women were indexed with their first and maiden names in reverse order to facilitate finding them when only their first name was known. And I’ve seen other variations. Indexes to marriage records may have a separate series of entries for grooms and brides, or the book may be alphabetical by male with a female index in the back. Names of bondsmen (if appropriate) may be in the book, but not indexed.
This is not always explained in the index.
Don’t assume that the index you have is always a strict alphabetical index–it may not be.