you have the same tarp as in the picture
did you actually set it up or are you just looking at it on the ground? (is a square or rectangle turned on point not also a diamond?) if you look at the way it's setup in the pictures (both on this thread and on my site), i think "diamond" is a fair description.
it is somewhat rectangular, but when turned on point, so 2 corners face the trees and 2 face the ground, it should be exactly like you see in the pics. there are 2 different ways to set it up as well. the two diagonals are not the same length, so depending on how you set it up, you could set it so it's slightly longer or slightly wider. (so it could appear setup correctly, but you might get better side coverage if you turn it 90 deg for instance).
it's a minimal tarp, and may not big enough for some. it's not going to provide full side coverage from wind. in order to cover all the way from the ridgeline of your hammock down past your butt, you'll need a min of a 9 foot wide hex or rectangle tarp, but there are alot of people who are fine with the coverage it gives and the weight they save by using it.
you can return it if you want, just let me know