This is a phrase oft-used by real estate agents. Their use of it typically implies a positive connotation i.e. a deceptively spacious house is really spacious, perhaps surprisingly so. But really this phrase means the opposite, doesn't it? Surely a deceptively spacious house deceives you into thinking it's spacious when in fact it isn't. The spaciousness is deceptive and hence not real.
I'm curious what people understand this phrase to mean.