Yeah "level" definitely comes from the concept of each stage being on its own level of a building or something, but it's still strange to use in such a general sense when levels are rarely actually on different levels.
"Level" in terms of RPGs is using the most common definition of the word. A level is supposed to be a number that represents the extent of an ability or attribute. It makes sense to say "level of strength" or "level of experience".
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_foolmo_
'You are obviously intelligent and insightful' - Sir Chris about me