I don't think this is all that strange, because even within the English language there's a difference of interpretation of what the "first floor" really is. British English uses "ground floor" for the floor that is at ground height and "first floor" for the first floor you need to reach by stairs. American English often interprets "first floor" as the first floor entered (the one on the ground). Of course, the British rule has left the American continent these days, but in other countries this could very easily become a problem.
In a country with different cultures, languages and systems of writing it's only natural that the Chinese representation reflects what a Chinese person would call that floor, with the English representation next to it converted to whatever level of language compatibility someone might choose for international mail.
It's strange in that my floor number changes depending on what language you are using. For systems that parse addresses into a structured format it's a real problem, as they need to ensure they are rendered into the original language.
In a country with different cultures, languages and systems of writing it's only natural that the Chinese representation reflects what a Chinese person would call that floor, with the English representation next to it converted to whatever level of language compatibility someone might choose for international mail.