The ABV calculation is not linear. Use the "alternate" calculator on Brewer's Friend to get a more accurate ABV calculation at that high of a starting gravity. Using your numbers it spits out 14.25%:
A more complex equation which attempts to provide greater accuracy at higher gravities is:
ABV =(76.08 * (og-fg) / (1.775-og)) * (fg / 0.794)
Also, I'm glad you're seeing numbers like this. I have plans to brew a 3 gallon batch of Imperial Stout soon that should be about 1.120 OG and I was hoping to get it around 14% ABV with Nottingham. I'm going to age 1 gallon on bourbon-soaked oak chips, 1 gallon on rye-soaked vanilla bean, and either leave the last gallon as-is, or age it on something else (coffee, cacao nibs, rye-soaked blackberries, pinot noir-soaked oak chips).