But if the beer has fermented, it has alcohol and alcohol is lighter than water. Specific gravity is the ratio of the weight of a given volume of liquid to the same volume of pure water, so you either have an extremely thin beer or a very high ratio of alcohol to dissolved solids like complex sugars and proteins. That's why I asked how it tasted: Some infections eat the complex sugars, reducing the gravity below what you'd normally expect. Extremely high proportions of sugary adjuncts can do this to a beer as well - if you used a lot of honey or cane sugar you'd get SG's lower than normally expected. Some Belgian yeast strains really dry out the beers, particularly in the presence of a lot of simple sugars. So I propose the ultimate test: Taste it and if it tastes good, drink it. It'll be very dry but that may be just the thing!
White Labs says it can come from fermenting at too high a temperature. I do it all the time unfortunately, and get attenuations around 90%. Beers good, but probably would be better if it hit its calculated attenuation.