This could be a case of drunk yeast.
If the yeast strain's alcohol tolerance isn't very high, it could have trouble fermenting any more because it's already in 6.5% alcohol beer. That's why they have special yeast strains for high gravity brews. This one isn't super high, but the yeast could still be a bit impaired. That doesn't necessarily mean it won't get any lower, but it's going to be harder and take longer. What yeast did you use?
But even if it does ferment out more, I don't think it's enough for bottle bombs. If you're carbing the stout to style, it's going to be carbonated on the low side, so there's plenty of wiggle room.
If you need me, I'll be out behind the woodshed pounding Grolsches.
Primary: Maris Otter/EKG SMaSH
Bottle: Deizbeutzervet Dunkelweizen
Keg: Independence Day Strawberry Blonde
Keg: Munich/Mt. Hood SMaSH