The gravity potential of fermentables is measured in points per gallon. This is expressed as the nonzero digits to the right of the decimal point. So, let's take a light pilsen syrup as an example. PPG for pilsen LME is ~36 or 1.036. So, at 5 gallons, if you had 5 lbs of pilsen LME, you would be at 1.036. You used 6 lbs of LME, so you have to divide 36 by 5 to determine how much that will raise your OG. 36 / 5 = 7 rounded down. You are now at 1.043. The grains added were probably somewhere in the 1 to 2 pounds range. If we assume 2 pounds of something common like Crystal 40L, that's about 1.005. So, for a 5gallon batch, you would be at around 1.048, which is roughly where you ended up. With a 4.5gallon batch, however, you would have been at around 1.054, which is much closer to where you should have been, according to the recipe.
With an extract recipe, you are in a great position to make adjustments, since you are not concerning yourself with all of the precision of an allgrain brew. Do a smaller boil, get your wort down to pitching temperature, then take a gravity reading and add preboiled/cooled water to get your wort to the OG that is correct for the beer you are making. You can always add water, but you cannot take it away.
I would also recommend checking the anticipated OG of the recipe against style guidelines to make sure it is not way off. Better safe than sorry.
