When it was minted it was issued as "prooflike", highly polished but not to a mirror finish associated with proofs. As noted above it got roughly cleaned sometime in the last 50 years.
At one time it just had way too much proof, so, some of it was given to another coin because it really needed it.
You can tell it’s been polished because the luster disappears next to all of the raised surfaces, which is where the wheel was unable to make contact with surface of the coin.