Original gravity is very important to the balance of a recipe. OG is essentially the sum of the alcohol and the residual sugar left in the beer. Both of these are commonly offset by hops. This is where the BU:GU ratio comes from. (1) If the OG is significantly low the beer will have less alcohol and residual sugar which will cause the beer to taste more hop focused. The converse is also true. Recipes are balanced for a reason. Tipping the balance makes a different beer, which might make a fine beer, but you probably selected or designed a recipe with a goal in mind.
So if brewers don't hit their OG right away, how is it achieved?
There are several ways this can be done. This is my plan of attack:
1) If the OG is high, then add more water. If the OG is low then add DME.
2) If the first suggestion can't be done, then scale the hops.
Adding Top off Water
Specific gravity is a ratio of dissolved solids (in this case sugar) to water. It can therefore be simply scaled. I find it easiest to deal with the number if they are in gravity points. For example if I was trying to make a 4 gallon 1.050 wort, but my gravity was 1.060 because of unexpectedly high efficiency I would need to add top off water.
4 gallons * 60 GP = x gallons * 50 GP =>
x gallons = 4 gallons * 60 / 50 =>
4.8 gallons are needed, so 0.8 gallons of water would need to be added to achieve the desired 1.050 wort. This, of course, makes more beer so the hops will need to be scaled as well. (See Scaling the Hops below)
On the other hand if the OG was low, the gravity will need to be adjusted up. Lets say the 4 gallon batch came out it 1.050 and the target was 1.060. The easiest way I have found to do this is to add dried malt extract. We need to simply figure out how many gravity units need to be added to the beer.
4 gallons * 50 GP = 200 GU (What we have)
4 gallons * 60 GU = 240 GU (What we want)
240 GU - 200 GU = 40 GU (how much we need to add)
40 GU / 46 pppg = 0.87 lbs
0.87 lbs * 16 = 14 oz.
Boiling down the wort
An alternative to adding DME would be to decrease the batch size by boiling the wort down. However, like when adding water, because the volume has changed you will need to scale the hops as well.
4 gallons * 50 GP = x gallons * 60 GP
4 gallons * 50 GP / 60 GP = x gallons
x gallons = 3.33 gallons would need to be the volume of the wort after boiling it down.
Scaling the Hops
Changing the amount of hops will make a different beer, but but it will still be a balanced recipe. In the event that the other methods cannot be used for some reason this could be a way to keep the beer from slipping too far from the intended style. We'll use the same example where our specific gravity coming into the boil is 1.050 but the target was 1.060. We'll scale the hops back to maintain the same BU:GU ratio. Let's assume that the 60 minute addition was 0.5oz of hops, the 30 minute was 0.75 oz, and the 5 minute addition was 1oz.
Scale factor = actual OG / target OG
50 GP / 60 GP = 0.83
bittering hops: 0.5 oz * 0.83 = 0.415 oz
flavor hops: 0.75 * 0.83 = 0.625 oz
aroma hops: 1.0 * 0.83 = 0.83 oz
(1) Ray Danials "Designing Great Beers"