You probably didn't have the water and wort mixed well before taking a reading. The heavier wort sinks and the water rises to the top so your reading is a little light. Don't worry about it- the yeast can find those fermentables! It'll be fine.
Next time, mix well and really whip it up to aerate it (after it's cool, that is!). Mine gets a white frothy head on it, then I pitch the yeast.