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This is a consequence of choice of coordinate system mapping directions. By convention OpenCV sets the left cameras as the world coordinate origin (0, 0, 0) with identity orientation, and the translation t is defined as

where C is the camera center in world coordinates. Basically this definition gives you the vector that maps points from the left camera coordinate system into the right camera coordinate system, i.e. points in the left camera system are translated by a negative x value when mapped into the right coordinate system, when there is purely x translational motion between the two cameras.