I found a statement on slimming world once in regards to this sort of thing. I think there have been some 'butchers' that have called things syn free, when they have not been, not Joe's sausages but other butchers. Slimming worlds explanation was saying that they do not endorse any meat products from butchers in order to protect weight losses of its members, and it doesn't allow any meat producers to use the slimming world trademarks on their sites.
That being said, i buy sausages from Joe's, and i have recently checked all the products that i get from Joe's, and they all come out syn free on the calculator. I think for the calculator it makes a difference on the nutritional information \ per 100g to the portion size sometimes, so it might be a mistake here that led you to the syn values you got.
Also i find with Joe's that they taste healthy, as is when you cook them no fat leaves the sausage, they are much firmer, and they do go quite solid if re-heated. This is simply because they have such a low fat content and therefore are almost solid pork and seasonings.
I personally prefer them because they feel much more like 'real' meat than any other fatty sausage, but each to their own.