sugar ferments completely.
Malt extract does not.
so, sugar will raise your Original Gravity, but can result in a lower Final Gravity.
Now that said...let me guess..you boil 2 gallons, then top off with 3 gallons of water in the primary?
If so you DID hit your OG. its very very hard to mix 2 gallons of wort into 3 gallons of water evenly. You get a non uniform solution, so your gravity reading is off.
You basically cannot miss OG in an extract recipe unless you don't add enough extract, add too much...or add too much/not enough water.
Extract has no efficiency like partial mash or all grain brewing...because the extract always gives you the same gravity. mashing does not.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10