Well, the Midwest instructions do say 5 gallons, right?
Extract brewing is very much predictable. If you've hit your final volume, then you've hit the gravity. You can't decouple the two.
1.052 = 6lbs DME + 5 gallons. End of story. If you have some specialty grains, its possible you've got more sugar than Midwest predicts.
Or your hydrometer is off, you've forgotten to adjust for temperature, or the thermometer you used in figuring out the adjustment itself needs adjustment.