‘Realness’ in this sense is not referening to the authenticness of the experience, but the accuracy of the signal that can be experience.
When you use a telescope to look at stars from earth, the image of the stars are distorted by the atmosphere. What you see is not the real image provided by the stars, but its image plus a distortion. In this sense, what you see is not ‘real’. When you get into space and look at the same star, it looks much sharp because the air is gone. In this sense, it is now ‘more real’, because there is less distortion.
In this sense, stars are more real when you look at them in space.
(If you see Los Angeles without smog, it isn’t the real Los Angeles anymore, but you can argue that because of the smog, you haven’t been seeing the real Los Angeles.)
But more interestingly:
When you are awake, your experience of the world can be clouded by mental disturbances. It is the feeling you get when you walk around the house, with time floating by, not knowing what you are really doing or caring about anything. Your sense of existence, and of the connection to the world can feel vague and detached due to mental disturbances such as stress, apathy, pretense, and passivity. Those disturbances can be removed in a dream (not that you can’t remove them in real life). When that happens, your overall experience in a dream seem more connected to the world, more alive, more true to yourself, and more exist. You feel more of your presence, as Fadem said.
In this sense, a dream can feel more real than reality.