JointTrajectoryAction position vs /joint_state positions

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I'm trying to replay a saved trajectory by using the joint_states positions and JointTrajectoryAction. However, for some of the joints (upper arm roll), the /joint_state accumulates all the movement, so its position is some arbitrarily large number (let's say 600). However, JointTrajectoryAction seems to work on a different frame, because if I issue a goal to position 600, it ends up spinning indefinitely despite the /joint_state claiming it's very close to 600. How do the joint_positions differ?