I brewed an extract summer witbier kit (wheat lme, steeping grains 8oz barley 8oz oats) coriander and orange peel. I added 1/4 pound honey malt to the steeping grains, and 1 lb of honey after the boil. I gave fermentation some extra time to account for the addition of honey, and made sure SG...
They might be leaving room for a few things:
1. Leaving room for cold water after the boil. Most beginners don't have a wort chiller.
2. They're probably assuming a lot of extract brewers don't have a full size brew kettle.
3. Leaving some water behind to sparge with.