Think with me. You can do it, just believe in yourself.
There's an extended battery for the GNEX. Fine.
The extended battery is thicker than the original one. Ok?
That's why it comes with a different battery cover than the original one, to compensate space. I am right?
Ok now. The tricky part. Think with me:
Thicker battery + different battery cover = Thicker phone.
Thicker phone is different from original phone. Cool.
If the back of your phone is now thicker than it was before, the cases made for the original GNEX... yes, yes, yes, you're close... just DON'T fit
Get it?