Why, when I have a runny nose, I can blow and blow until theres nothing more to blow out, but always can then instantly snort an uncomfortable amount of loose phlegm which invariably gets caught way back in my throat. Why didnt that get blown out? Its not a question of it not being loose enough, If I blow my nose empty, I could continue to blow for an hour and produce nothing. Then one snort, and <gag>.This is a serious question, not just a whine!
All healthy humans have a continuous column of mucus running down their throats. We need it to coat the lining of our stomach so it won’t digest itself. The snot in the back of your throat is due to this natural flow of things. When there’s an infection in the sinuses, we send in white blood cells to fight it, and excess mucus to flush it out the nose.
At least we aren’t cows; they don’t send enough mucus to their noses, so they have to stick their tounges up their nostrils all day long.
If we all had tongues long enough to clean out our nostrils, we’d be a much happier species. 
The nose congestion you are feeling is not coming from excess mucus accumulation, but rather from swelling of the tissues inside the nasal cavity. It is sort of like the way a part of your body swells after blunt trauma. The best way to get proper breathing again is to leave it alone or take in some medicine (like night/dayquil).