Other meanings are harder to pin down. The estate agent's favourite, "deceptively spacious" - does it mean a property which looks small from the street, or from photos, but is actually very large? Or does it - as Dogberry thinks - mean a property which is rather small, but gives the impression of being spacious through use of light and clever decorating? Either way, it means a property whose spaciousness is compromised in some way - not very desirable, but perhaps intriguing enough to persuade a buyer to set up a viewing.
And some usages are simply baffling. What does The Times's description of a cricketer's bowling as "deceptively effective" mean? That it was more effective than it looked? Or that it was effective because of its deceptiveness? Similarly, I quite can't work out what the "deceptive" is referring to in The Independent's description of "deceptively well-crafted poems" or The Times's of "deceptively well-made" short films, although I suspect the meaning in both cases is something close to "deceptively simple". The most marvellously mysterious, though, was the reference in a Times music review to Deep Purple's "deceptively funky cymbals". It is clear from the context that this is a good thing, but beyond that the meaning is entirely lost on me.
It seems that the advantages of versatility and conciseness (it's quicker and neater to say "deceptively shallow" than "shallower than it appeared") overrule any concerns about confusing the reader. "Deceptively" also allows the writer to lay claim to a degree of perspicacity in discerning what is not immediately apparent; this may also be part of its appeal.