Learn English – “I won’t stay longer than I can help” or “longer than I can’t help”

In theory, this should be easy. Drop "deceptively" and you have the essential quality of the subject:

"The floor is deceptively flat" → The floor is flat.
"He's deceptively strong" → He is strong.
"He looks deceptively strong" → He looks strong.

Now add "deceptively" back in to indicate how the observer has been deceived:

"The floor is deceptively flat" → The floor is flat (but appears otherwise).
"He's deceptively strong" → He is strong (but appears otherwise).
"He looks deceptively strong" → He looks strong (but is otherwise).

The difference between #2 and #3 is the difference between "looks" and "is." This has the effect of flipping the comparison around: in #2, you are deceived about how he is, and in #3 you are deceived about how he looks.

In theory, it's easy. In practice, you're just as likely to find the word used to exactly the opposite effect, so in most cases you should assume you're going to need to get the meaning from context. I'd recommend staying away from "deceptively" entirely when writing: "The floor is flatter than it looks."

In short, the word "in" has multiple uses.

• see that dress in red - See the same kind of dress, but with red fabric

• see the Eiffel Tower in person - Visit the Eiffel Tower yourself instead of looking at photos of it

• The business was in the red - The business has no money and their balance sheet total is in red ink signifying a loss

• He was in the pool - He was physically located within the pool

None of these are the same meanings of the word "in". They are similar but not identical. In some cases the prepositional phrase "in blah blah" modifies some parts of the sentence, in other cases it modifies others. Context lets you decide.

• I saw that coffee table in red - Is the coffee table red? Or were you wearing red clothes when you saw it? Both interpretations are possible, but only one is likely.