I'm sure every table is different in how they run their games, so I don't discount your quicker experiences. But I know that in the times I've played that if we try to play a full game that requires the victor to score 18 supply centers, we can't get that done before getting up into Years 1908, 1909, or longer (so we're talking 16, 18 even 20 or more total game turns). Which means with a 30-minute first turn for negotiations and at minimum 20-minutes each additional turn (15 minute negotiations plus at least 5 minutes to write and execute orders, but in truth that ends ends up closer to 10-minutes each turn)... that's 3 turns per hour if you're lucky. So about 6 hours of just game, plus then figuring in bathrooms, eating, and general conversation (which the game does say you aren't "supposed" to do, but really who can't?) 8 hours has been a standard.
That being said... I agree with you that when I've played it at cons the cons regularly constrict the time and end games much quicker (4 hours being a normal place to end). But usually at that point we only have someone at probably up around 8 or 9 supply centers and they get handed the victory (or the top two who are in an alliance are granted the victory but even they combined haven't reach 18 supply centers.)
Now admittedly my experiences could be the anathema, that's entirely possible. Maybe we're just good at being efficient.