We had a butcher shop for years and the meal that all butchers put in their sausages (wether good quality butchers or not) contain fillers
eg maybe rice wheat or some sort of grain, which do contain carbs.
This is taken from a label of such a meal for beef sausages.

Beef extra body (meal)
Ingredients Rice flour, wheat flour, salt mineral salts (451 ,450)
Preservative (223) Spices,Spice extracts, Hydrolysed vegetable protein, flavours
Caution: Contains not more than 1.15 of the preservative sodium metabisulphite
Contains Wheat gluten soy sulphite.
The directions for this are
beef and beef fat 37.5k
Beef extra body (meal) 4.5k
iced water 8kg
total of mixture is 50k
As you can see is about 7.6% of the total sausage. Quite an amount really.

Of course there are certain types of sausages that are lower in carb counts then others.

We now make our own sausages with just meat (beef goat pork chicken) with herbs and spices. They turn out quite nice.
I did a serch on the internet and found some recipes for sausages and smoke sausage and that is what we now use.
Hope this helps