Temperature also has a lot to do with it when you take your reading. Be sure to include that in your calculations.
That being said, if you mashed too warm, you will wind up with more unfermentable sugars in your wort. You could also have a stuck fermentation due to lack of available nutrients and oxygen (did you add those to your wort when you pitched the yeast?). You'll have to analyze your process to figure out what the most likely scenario is/was.
If you're bottling, you might try to use the minimum amount of priming sugar to the beer (especially if a stuck fermentation was the issue). Use strong bottles and caps, such as belgian beer bottles, bomber sized or 750 ml bottles. They're a little bit stronger than 12 oz. shorties and better able to withstand bottle bombs.
If you use kegs, no worries. Force carb the beer for a week at 30 PSI and if it's too carbonated, burp the keg a few times a day to free the extra CO2.
