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Revolute2 vs Universal joint?

I'm having difficulty understanding Gazebo/SDFormat's differentiation between a Revolute2 and a Universal joint.

According to ODE's documentation, a Hinge2 joint (which SDFormat/Gazebo call Revolute2) is actually supposed to have 3 DOFs: a rotation and translation around/along axis 1, and rotation around axis 2. It appears that Gazebo/SDFormat revolute2 joints only implement 2 DOFs, and ignore/constrain the translation along axis 1.

A universal joint, according to ODE, is just a Hinge2 joint without the translation DOF along axis 1.

So it seems that in Gazebo/SDFormat, a universal joint and a revolute2 joint are actually exactly the same joint (with a universal joint just being a particular configuration of a revolute2 joint).

Am I missing something here?