Symmetry is a, ah, well. Let's say that in the textile world... I think it's special.
Most of the things that we do, our basis is... yarn. Spun yarn. And now you probably already know where I'm going at: As soon as there's twist involved, or diagonals, there is no such thing as true symmetry. The twist in the yarn will always influence looks and behaviour of your piece.
In sprang, there's an additional element, because it's inherently symmetric - you are braiding on stretched threads, thus everything you do adds to your fabric twice, once on top and once on the bottom of the frame. Only... in the opposite direction. Which means you have the basic difference you get when you work with the same yarn in different diagonals.
Plus then you have the effect that you can place a heavy weaving sword at the bottom, while you are working at the top. Or even if you are not placing anything in your shed, you're usually working at one end, close to the fell, and that will tend to mess with the tightness of your previous braiding a little. Finally, if you have the top in a nice working position, the bottom might be in an awkward place, so you are doing your tightening with less useful visuals.
Long story short: It's surprisingly easy in sprang to end up with two pieces that are not the same size.
Which also means that I was pretty chuffed about how my two pieces turned out. If you like, you can try to guess which one was the top, and which one was the bottom bit in the frame!