Sorry to say, I can't see any other explanation other than you made an error somewhere.
Unless you added water somewhere the OG should have been higher than your pre-boil gravity because you concentrated the sugars during the boil.
Am I right in thinking that you started your boil, took a sample to determine preboil gravity, added sugar, boiled some more, cooled and then took your sample to determine your OG? If that's right, you added sugars and removed water (unless some of your cooling water made its way into your wort?).
Perhaps the problem was that, "I wasn't drinking at all during this brew session..." Always a recipe for disaster for me.
Whatever you did, you at least have an OG reading.