Imperial stout OG

I just brewed an imperial stout from extract. I used 12lbs of extract and 3.5lbs of steeping grains. The recipe states the OG should be 1.089. My specific gravity at 74F was 1.132 even after adding an extra half a gallon of water to total 5.5 gallons. Three weeks later the FG is 1.020. The beer definitively has a strong alcohol taste, but still tastes too sweet. I am wondering how I got such a high OG? Also wondering if the fermentation is stuck because it tastes still a little sweet. The recipe states the FG should be 1.020.