There's a slip between the input and the output of the Gamemaster-In-The-Middle. The adventure writer communicating to the GM says "This appears to be a worn stone stairway leading down, but really is a sloping passage floored with the sticky illusion-casting tongue of a Deceptive Devourer, the rest of which lurks in wait in room 15 of the next level." The GM then communicates to players, "You see what appears to be ... an old set of stone steps leading down into the darkness." Or consulting the "appears" synonym book, "seems," "looks like," "apparently is," et al.
Who knows why they do this, but two reasons come to mind. It could just be literal-mindedness, relying on the words in the description to craft a speech to the players. It could also be a reflex of honesty; the inner moral angel balking at saying there "is" a flight of stairs leading down when it just isn't true. Whatever the reason, it becomes immediately clear to the players that using "appears"-isms in this way is a giveaway that something funny is up.
Now, there's still time for you, the GM, to repent of your folly. Realize that your job is only to describe reality as it appears at any given time to the players. A successful deception will appear with the full force of reality; "is," actually, is fully appropriate.
But in some games I've seen, the GM instead takes the left-hand path, doubling down on "appears"-ism by applying it as a decoy to things that aren't deceptive at all.
"What seem to be some mushrooms are growing from the dung heap." (They're just mushrooms.)
"There are some humanoids approaching. They appear to be orcs." (And they are.)
"A stream of what looks like clear water flows from the left wall to the right." (PSYCH! It's acid, save or take 4d6!!)
In any case, "appears"-ism usually gets left by the wayside when the players enter safe surroundings. Or at least imagine this:
You find what appears to be the same trail leading back to the village through superficially familiar birch and fir trees. After walking a distance that feels similar to the distance you took to get there, you see what may very well indeed be the buildings of the village. You go to a low house that looks very much like your inn. A hot meal for five is seemingly brought out within what feels like minutes by the self-styled innkeeper, who closely resembles the man you remember from this morning. Pewter-look plates apparently are sitting on what looks like a table, with a liquid having the appearance of ale in a ceramic-like pitcher. The "plates" are heaped with putative sausage and ostensible beans ...This, I think you'll agree, is a Brechtian alienation effect gone too far; it turns the game into an exercise in Plato's Cave or radical philosophical solipsism. Whenever appears-speak is used, it will keep the players vaguely tipped-off and on guard, lending a hallucinatory aspect to the proceedings.
But I'm not sure it's necessary to use such a blunt instrument to get that effect -- shouldn't players naturally be wary in the dungeon? And more to the point, how do you really spring the classic "innkeeper-is-a-werewolf" surprise when you telegraph safe and dangerous areas so obviously?
In conclusion, there can only be one response to an environment described through "appears"-book-isms ...