Have you calibrated the hydrometer?
+1 - that FG reading sounds way too low to me. That's the ballpark for something like a Saison or Brett-aged beer, but doesn't seem right for a dubbel. Even if you mashed super low and used 20% simple sugar, I can't imagine most Belgian yeast strains finishing quite that low. If your hydrometer was off by 10 points or so, then I think you'd be in the right ballpark.
A few followup questions for you:
- How did the hydro sample taste? If it was truly that low I'd expect it to taste hot, thin and boozy.
- What was your grain bill/mash schedule?
- How much sugar did you use in the recipe?
As far as whether 99% attenuation is possible, it is with something like a Saison strain or Brett. But keep in mind that this is apparent attenuation. Alcohol is lighter than water and will affect how your gravity reads. that's why things like ciders and meads finish out below 1.000. The sugars are 100% fermentable, so the amount of alcohol present will bring your FG reading below one. In your case the real attenuation is closer to 80%, even with a 99% apparent attenuation.