1. The centers of rotation aren't fixed, i.e. when you see the side profile, they're much closer together. I think this makes it easier to compare them side-by-side, especially when trying to compare thinness.
2. Rotation is restricted to only two axes (vs. all three). It's surprisingly difficult to control a 3-axis gimbal with only 2 axes on a screen. For those who aren't familiar with 3D CAD/modeling, try orienting this gimbal simulation [0] in an arbitrary angle (or doing a barrel roll). Restricting it to two axes also keeps the models from going into orientations that probably aren't very useful for the user.
I don't know the first thing about "3D UX," but I thought they were good, albeit subtle, interface decisions.
[0] http://www.ctralie.com/Teaching/COMPSCI290/Materials/EulerAn...