Generally I think it has to do with movement. The background images while they are moving if the screen is panning, are stationary.
The fish are all over the place in a purely random fashion. You can't predict where they will move to next, and therefore the 3D software can't either, so it is always slightly playing "catch-up". Granted this is done in microseconds because of the computer nature of the beast, but it is still doing it, with a ton of polygons to have to deal with, so I think random moving objects sort of stress out the properties of the 3D effect.
At least that was always the way I thought of the process.
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