From “Gilb’s Mythodology Column: User Stories: A Skeptical View by Tom and Kai Gilb”, an article from the AgileRecord.
Full PDF download here: http://www.gilb.com/tiki-download_file.php?fileId=461
Myth 2: User stories represent a common language. They are intelligible to both users and developers.
User stories are not necessarily intelligible to all users, all devel-opers, or any of them.
In fact it is very easy to prove that user stories are normally NOT intelligible.
You can use the same way to prove that a specification is normally NOT intelligible. And yes, there are specifications that are not good enough and have many ambiguities.
The goal of the Backlog Grooming (Scrum Guide) is to fix these ambiguities and to prepare the user stories for the next sprint(s).
Splitting a user story is a common practice there. I never heard of splitting requirements…