My guess: it isn't wear on the wood - it's wear on the paint. Many inexpensive guitars have painted fretboards - that way they can use really cheap wood, but make it look sort of like ebony. It's even more common on nylon string guitars, at least in my experience.
The fact that it's 'wearing' between the strings is the big clue. Your fingertips are slowly rubbing off the paint. Fingertips are rough enough to do that - but not really hard enough to wear away the wood.
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