My guess here is that it is the actual gravity of the wort you are boiling. So if you add 5 lbs. of extarct to 3 gallons of water the gravity will be higher than if you added 5 gallons of water. It's probably for utilization purposes. Make sense?
I used to calculate out what my predicted OG would be based on my ingredients. Then I would calculate the IBU using the predicted OG, as it affects the overall IBU.
I have never taken a pre-boil sample gravity. I'm sure there are good reasons for it, but I don't bother. If the recipe I'm using has a predicted OG of 1.05 and with the hops I am going use will get me 30 IBU, I just follow it through on brew day. If my gravity of the wort, just before I pitch is slightly off (a few points higher or lower than the predicted) then I figure I'll be fine. I'm not sure how strong anyone's tasetbuds are to detect a slight shift in IBU from not hitting your OG exactly - as long as your have followed the recipe. Don't get me wrong, if you miss your OG by a lot (1.03 vs 1.05) there will be a difference! Also, don't me wrong in that I am advicating not taken readings, becuase the gravity readings will tell you alot about what is going on with your brew.
Also, I finally broke down and bought brewing software to do all the calculations, best money I spent this year so far!
That is how I take it. There ought to be a way to predict your gravity based on the boil size and extract addition without actually taking the gravity. What is your boil volume and extract addition? Be patient as I'm sure someone will come along that may know more about the program.
I'll probably be using 3.5 lb of Maris Otter LME and 1 lb of light DME along with some specialty grains. My "Joy of Homebrewing" book as the formula for figuring this out. Perhaps it is just better to do the math the old fashioned way.
Look on the bright side, its a good way to understand formulating recipes for gravity and bitterness!! My last brew was my first software (Beersmith) run. I did the previous 10 brews by hand calculation!
BYO.com has a good list of gravity pts for grains, extracts and adjuncts.