I say this as someone who's a fan of big, complicated, hours long games; 90% of the time, the numbers listed on the box are a total fucking lie.
OK, to be more precise, they typically measure how long the designers - who know every rule, never deliberate over what action to take, always pass priority when they're done, and never stop to grab a glass of water - think the game should take. And probably don't include set up time.