here's the dilly:
"Starting Gravity" is not a "product" of anything, per se. It's just a number, a reading taken at a certain point. That reading, as you know, is taken after the boil, prior to fermentation, and it measures the amount of sugar that is dissolved in the water. That's it.
Holding the initial wort volume constant (5 gallons for most), the boil has absolutely nothing to do with your gravity. Of course, if you start out with 8 gallons of wort, boil it for 60 minutes, then you'll have boiled out alot of water---but no sugars---meaning your gravity will increase because the sugar/water ratio will increase. But it ALL depends on the volume of the wort.
For example, let's say, pre-boil, that your wort volume is 8 gallons, and you have a specific amount of sugar in that wort, call it X. You boil for 60 minutes and end up with a 5-gallon wort. You take the gravity of that final wort, and it's 1.040.
However, let's say that, instead of starting out with an 8-gallon pre-boil wort volume, it's 5 gallons, but with the same amount of sugar, X. You boil for 60 minutes, end up with 2.5 gallons of wort, then add back 2.5 gallons of fresh water to bring the final volume back to 5 gallons. Your original gravity post-boil should be EXACTLY the same, 1.040.
Long story short, if you're trying to adjust your original gravity, there are two variables that you can adjust: the amount of sugar in your wort (pre- or post-boil), and the amount of water in your post-boil/pre-ferment wort. When doing AG, you increase or decrease the amount of sugar molecules in your wort by adjusting the MASH SCHEDULE and the ingredients therein.
If you feel that your OG was too low, there are two possible culprits: too much water, or not enough sugar. Assuming that you've adequately measured and controlled the amount of water, that leaves the mash...
So what was your mash schedule and your grain bill? Low efficiency can be caused by many things; you should be striving for at least 65-70%, though.