[benwr]

Entropy of a single password isn't actually a well-defined concept; entropy is always about a distribution. "Entropy calculators" that look at your password and tell you "its entropy" are making assumptions about how you chose the password.

We care about the distribution from which you drew the password, because that lets us analyze how difficult it would be for an attacker who knew your password selection process to brute-force the password. Just knowing the password itself isn't enough information to determine that (though of course you can judge how hard it would be for an attacker once you know their brute forcing strategy).