First floor plan of single-family house by architect

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Why is the hallway on the upper floor nowadays always called a gallery?

....:)

Not only nowadays and not always: it is called a gallery when the hallway opens into a room on one side.

that is called a gallery when the corridor opens into a room on one side.

That's how it is (actually), but I think the question was really meant like this: why it is often called that today even when it is actually a simple hallway without any valley view into the ground floor. Simply because it sounds more valuable.