It's just the preview image. It looks like everybody else made their previews use the double pack, while I used the triple pack for my previews. If you actually open them, though, the spine is the same size and it has the same dimensions.
We seem to be using different versions of Imandix - mine is the free version, but it doesn't leave that 'IMANDIX COVER TEST VERSION' at the bottom, so it is probably lacking whatever features they are taking advantage of.