Right. To clarify, Drummer is in the book, but the role Drummer is currently playing on the Behemoth were different characters in the book (she basically took 1, potentially 2, roles from the book in terms of her current duties on the ship. In the books, Drummer is still back on Tycho/the Solar System).
It isn't too surprising. In a book it is fairly easy to introduce tons of characters, but in a screen adaptation, it can make sense to merge characters so you don't have too many people to follow (or actors to hire/retain).