9 pounds of DME in 5 gallons of water will make a wort with a gravity of 1.079 every time. If you added that much extract to your recipe, you will have a beer with about 8% ABV. Chances are your extract was not fully mixed into the wort, and your hydrometer sample was slightly watered down. Dont worry, it is impossible to "miss your gravity" with extract. It is very common to have inaccurate hydrometer readings with extract, though.
"Give a man a beer, and he will waste an hour. Teach a man to brew, and he will waste a lifetime!" Bill Owen