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I have the exact same thing. Only if I press the top left corner, there is slight slight movement. You can tell there is some spacing, but it's not obvious or that noticeable unless you looks real hard. It's very minimal and I don't think it's significant at all.

It looks like it just might be from production. My phone has been in my case since day 1 and have never sat or have it in my back pocket at all. It has no effect if I just lay it flat - it does not wobble when in use. Only occurs if I press with some force on the top corner to show some minimal "wobble."
 
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