There could be a few reasons, saddles are usually made on a fibreglass tree but also wooden ones were used and still are....depending on how old and where you saddle is from.
Some saddles made in India for example are very much knocked together, trees that are already split, flocking made out of waste...even old socks!
Some people buy these cheep saddles off the net, thinking they are ok, better quality leather than what they used to be from the outside, but if you took it to bits, you'd be in for a suprise!
Not saying that is the case here, no idea what saddle you have, just giving one example of why you could end up with a damaged tree.
I'm not a saddler so hopefully you will get much more qualified people answering your post but my understanding is that its VERY possible to have a twisted tree. In fact I've been told that its not unheard of for some 'budget' saddles to start life with twisted/mishapen trees.
My 13.2 twisted two saddle trees - by generally being a div! She wasn't very straight and kind of bounced and skipped about a lot. Both her saddles after a few months use were a good bit twisted and flipping useless.
To be honest you can buy a well known make of saddle - and not a cheap one at that and find it has been made on a non symmetrical tree. Most saddles are made with wooden trees, although now there are more and more synthetic trees out there which should be made symmetrical and impossible to twist. However the wooden trees are finished by hand so human error plays a large part in the finished article.
If you buy a saddle with a wooden tree and continuously get on from the ground you will eventually twist your saddle tree with all the pressure continuously on one side.
If you ride lopsided you can twist your tree. If your horse falls on the saddle you can twist your tree.
Howevr I have seen saddles with a so called twised tree which is actually nothing to do with the tree and all to do with being completely unballenced in the flocking and others which look twisted because the leather has been stitched on incorrectly and panels are not alligned correctly.
I brought a brand new saddle some years ago for a new horse, had my master saddler check after 6 months and the tree was twisted he said it had probably been done when the saddle was made and it was quite common. He took saddle apart, straightened the tree and hey presto one perfect saddle.