Or on the contrary they contain discrepancies?
Not discrepancies, just different suggestions of what might have happened. Over the decades, Trek tie-in literature has presented multiple alternative versions of many key events in Trek history. Pocket Books and DC Comics published distinct versions of Kirk's first mission as Enterprise
captain within a year of each other back in the '80s. To date there are at least seven distinct versions of the end of the TOS 5-year mission. There have been a few different interpretations of McCoy's divorce and even the names of his wife and daughter. There have been two or three different versions of Kirk's Kobayashi Maru
test, not counting the alternate-reality version in the '09 movie. There have been a few different versions of the Earth-Romulan War. There have been multiple different theories about the Mirror Universe's origin and history. And so on. While there are ongoing tie-in continuities here and there, it has never been a requirement that all tie-ins interpret offscreen events in the same way. There have always been multiple options available to the readers.
I like to say that if you treat screen canon as Trek "history," then tie-ins are historical fiction. They're conjectural works, positing how unseen events might
have happened. And different writers of historical fiction can come up with different conjectures.