Yes exactly, and according to Anandtech the power draw is actually only about 20% higher while playing Zelda (8.9 watts portable vs 11 watts docked), so even on the stock battery it's shouldn't be a massive difference, Say 2.0-2.5 instead of 3 Hours.
Isn't the one thing people are missing is that the screen is off when docked having the display output by an external device whereas when undocked the screen is in use using more power. Not sure how much a difference that makes but it seems to have not been mentioned yet, unless I missed it.