Learn English – Long-standing research question

Mulling takes an indeterminate time. If you're mulling wine, you heat gently, and add spices; it might take twenty minutes. If you're mulling over a life-changing decision, you might take several weeks, possibly longer. Anywhere in between might be counted as mulling.

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."