FG of bottled condition beer lower than FG of fermented beer

Question

I brewed a session stout about 4 months ago, and left it in the fermentor for two weeks. The FG was 1.012 one week in, and I checked every few days until 2 weeks in, and the FG remained at 1.012. I added enough corn sugar to add 2.0 volumes of CO2, and spent 120 seconds stirring it slowly into the bottling bucket. The bottles have been conditioning at room temperature during the winter.

I pulled out a bottle the other night and found it to be too carbonated. I poured some into a test jar and waited 3 days before measuring its gravity to ensure that carbonation wouldn't alter the results.

The final gravity of this sample was 1.010.

I'm straining to see the cause of this. The FG remained at 1.012 for a full week before bottling, the fermentor was under perfect temperature control, and I used an adequate yeast starter, so it is hard to think that the yeast did not attenuate fully. I spent 2 minutes stirring in the corn sugar, so it is hard to think that the sugar was not evenly distributed.

What could be the cause of this?

Vitals:

9 # Marris Otter
1 # Roasted Barley
OG: 1.047
Wyeast 1028, Yeast Starter 1.5L 1.035
East Kent Holdings 1.75oz 4.5% AA
Fermented at 65*F in a chest freezer, raised to 70*F after four days
Time in fermentor before bottling: 2 weeks
Final gravity prior to bottling: 1.012
Final gravity 3 months and 2 weeks after bottling: 1.010

Answer 1

Corn sugar will add a bit of alcohol, which will affect your reading. That is my guess as to why the FG in the bottle is lower.

Answer 2

Double check all your gravity readings and measurements of priming sugar, considering temperatures.

The SG's you noted could be off If temperature wasn't factored in the hydrometer readings.

The over carbonation would suggest an overdose of priming sugar, your 5gal batch at 68° would use only 3oz for 2.0 co2 volumes for example.

Or the bottle conditioning was warmer than your secondary allowing the yeast to be more active and attenuating more than just the corn sugar.