How is that unique to BW, or Narrativist play in general? In PbtA the game will spell out its aims and describe how the GM decides what to frame into a scene. This is important!
In many other games out there that I've seen, If the GM doesn't specifically describe something, but the thing would logically be present and not hidden (accidentally or purposefully), and if the PC asks if that something is there, the GM will usually say "yes."
As I have repeated before, the wounded guy was put in a room to recover. Pemerton has never once objected to my use of the word "sick room," and the word
recover seems to suggest that he was treated, so we can gather that he wasn't just dumped on the nearest flat surface and left to rot in his own juices. Thus: cups, bowls, jars, and jugs to hold water, herbs, and medicines, a chamber pot (which pemerton even mentioned), and so on. Thus, it is logical for the GM to say "yes, there's a cup right there."
I can also see the GM saying "but there's an axe-wielding assassin in the way, so if you want to get past the assassin and go straight for the cup, then roll Agility." (Or whatever the actual skill or stat used is.) Pemerton said this was a possibility if he hadn't instead called for a spot check.
I can even see the GM saying "you're not sure because there's an axe-wielding assassin who noticed you entered the room and is now moving towards you and raising their axe to strike" and
then saying that if the PC wants to split his attention between the assassin and the room in general, then they can roll to spot something. Pemerton never said this was the case, or if he did, it was a thousand years ago and I don't remember any longer.
When the PC came in, the assassin had killed the guy but didn't seem to have trashed the room. Meaning that those cups, bowls, jars, and jugs haven't been strewn about or covered. If the room
had been trashed, it would have been logical for the GM to say "The room has been trashed and everything is a big mess. Roll to see if you see a cup."
In some games, the PC can just say "I grab a cup off the side table" and
ta-da, now a cup and a side table exist and always have, possibly even if the GM has a logical reason why there wouldn't be.
Some
other games allow the
player to make a roll or spend metacurrency to add a detail the the GM didn't include:
GM: <describes room>
Player: Is there a cup there?
GM: Do you
want there to be a cup there?
Player: Yes, so I'll spend a point.
(Whether or not the appearance of a cup is worth spending a metacurrency is a completely different discussion.)
So: there's the difference.