In real life we expect things which are farther away from us to be spaced closer together than things near us, like the plates and legs on this table. Look at the plates with using the cross-eyed free viewing technique. Now in 3D, the plates on the far side of the table appear to be farther away than the plates on the near side.

But now look at the scene using the parallel free viewing technique. The plates on the far side now appear to float above the table. Your mind is confused. Your perspective of the scene is strongly suggesting those plates are farther away. The binocular disparity informs your brain they are closer than the lower set of plates.

When all cues related to perspective are absent from the scene, as below, only binocular disparity determines the depth effect in the stereoscopic view. Either technique can be used. When using parallel viewing, the center row clearly looks closer than the outer rows. Just the opposite is experienced when you cross your eyes.