Basically if your horse has a hollow behind the shoulder below the withers. It's fairly typical conformation for a lot of thoroughbreds, warmbloods and draught types or crosses. The trapezius (triangular shaped muscle running laterally along the withers and down over that area of the ribcage) is not well developed. Plus it's one of the areas horses carry body fat which tends to disperse under pressure.
The saddle gullet (ie. Both legs of the front arch) must run parallel to the back of the shoulderblades to avoid any pinching as the latter rotate backwards when the horse moves, but if the horse has these hollows and the saddle panel's not the correct shape or not flocked properly the saddle will drop into the hollows. This can make it seem apparently too wide or too low on the withers and if ridden on it will tend to throw the rider's weight forward. The altered angles can also make the saddle too tight in front, the cantle may lift off the horse's back and possibly it will rock front to back, especially in trot.
The other thing that may be more noticeable if the saddle sits like this is it goes to one side. Horses are all one-sided to a greater or lesser degree, same as us, and if the saddle is tight the bigger shoulder has a proportionally greater effect on how it sits and will often force the front over on one side.
Sorry - that went on a bit. Hope it's understandable :)
Thanks for the explanation, it makes a lot of sense and I can visualize exactly what you mean. I might take some photos of Mitch without a saddle on when I do put the saddle on. (Hopefully tomorrow but we also have a full day of Jet Sprints, first round of the series!)
I always wondered how the likes of showjumping saddles especially, seem to quite commonly lift off the horses backs during sj rounds, even when they weren't going over a jump at the time, that makes a lot of sense.
Ah, that is one thing I had noticed already with my dressage saddle and my gp. (The two that are now in a saddlery being sold) they both sat seemingly straight over the front of the saddle, but looking from the back of the saddle, both of them sat further over the right side, with the left panel going so far over as to actually sit on top of the back of Mitchs spine.. Even to a completely novice eye I knew that wasn't good, but wasn't sure why it was doing that, so out they went.
I know all this is only temporary until I have my saddle fitter out in the early new year, but I find all of this really fascinating, how many different things you need to check and change, how many different factors can actually affect a saddles fitting and performance on the horse. I also just researched how to find where the saddle shouldn't go past, and found out how to find the T18 vertebrae, I'm mainly concerned about that for DJ, being only about 14.3hh-ish, I know this saddle won't go that far back on Mitch