Well that's tremendously embarrassing... Thanks!

If I make a recipe for a 1 gallon batch with 1 gallon boil and 4 lbs of extract, it gives an OG of 1.052 for an ABV of 5.1%. This is clearly wrong, and doesn't even match with the Extract OG calculator elsewhere on the site, which gives an OG of 1.180 for ABV of 17.01%.