Why Icelandics are hard to fit--Wither tracings across several breeds

On a discussion group for gaited horses, somebody asked about the challenges of fitting an Icelandic.  I went to the stable and took some wither tracings across a variety of breeds.  The good news is that I came away from the exercise with a greater appreciation for knowledgeable saddle fitter/makers; the bad news is that Blessi, despite maintaining his weight has thickened up through the withers again so I am going to have to re-evaluate saddle fit.

From top to bottom:
- my 18 year-old, 14.1 hand Icelandic imported from Iceland.  Measurement 3 inches from top of gullet is 9.5 inches.  This tracing illustrates the challenge of fitting Icelandics.  Like many draft/pony breeds such as Fjords, wither shape is usually an upside down bowl with no indentations (usually) for the withers.  Only a few English saddle makers such as Duett and Thorowgood make saddles for these rounded backs but even they have slight indentations for the withers.  There are even fewer English saddle makers who make a true "hoop" saddle.

Blessi's wither shape is typical of the Icelandics.  At 8.5 inches, he was above average width for an Icelandic.  At 9.5 he is well above average and he weights about 980 right now (lowest weight was 920).  When I was looking at off-the rack Western saddles, only the draft saddles were wide enough through the gullet but nothing else about the saddle fit.  The sides winged out so far that Blessi looked like he had the Flying Nun's hat set on the middle of his back.

- This tracing is for Rocky, a 5-year old, 18 plus hand Clydesdale.  Measurement is 11 inches.  Rocky wears the Icelandic style saddle (as opposed to an English or Western saddle).  And I really did measure one extreme outlyer of an Icelandic who is 13.2 hands and measured 12 1/4 inches.  His back was so flat you could set plates on it.  He ended up with a 36 or 38 cm Duett saddle.

- Thor is an 8-year old, 15.3 hand mustang.  MtDNA analysis indicates his genetic heritage is 98 % Iberian (his herd was really isolated).  Measurement is 6.5 inches.

- Raffiki is a 5-year old, 17 plus hand Dutch warmblood.  Measurement is 8 inches.

- Sunny is a 12-year old, 16 plus hand Quarterhorse.  Measurement is 6 3/4 inches.  He hasn't been worked regularly now so he is not in as muscled up as normal.

So my experience with saddle fitting for Icelandics may be limited to those I have met.  Other Icey owners, do you agree with what I have found?  Or do you have different experiences with saddle fit?