Your query letter, and subsequently your synopsis, set up expectations for the reader. If you mention a hook, minor characters, or climactic points in these two the manuscript has to deliver these expectations. If there are inaccuracies they are often due to writing the query or synopsis based on your original outline. Or, writing a query or synopsis for the book that you idealized to write, not the product you are delivering into an agent or editor’s inbox. Conversely, go back to the manuscript and pull out the essence of the text (the key themes, characters, questions) and be honest with yourself and your reader. Also, it does no good to mention trendy themes, buzz words, or tack on genres that are not delivered in the sample material.
These issues can be solved in two ways: research and objectivity.
Writing a query or synopsis should be the very last thing you do! Firstly, with each manuscript revision you make you won’t want to keep going back and changing your query or synopsis. In that case you may miss phrases in the query or synopsis that should have been changed to fit the new manuscript, but are left in because the difference in drafts becomes a blur. Secondly, give yourself time away from your manuscript before you go back to write a query or synopsis so you can look at it objectively, commercially and systematically.