As always the devil is in the details:

All of the aforementioned assertions assume the passwords are generated by a cryptographically secure algorithm and not stored in a reversible-encrypted or clear-text format.

If we are talking human generated passwords or passphrases of any length then they tend to be predictable and that formula for calculating entropy does not apply to them. To quote one cryptographer:

People who haven't worked on password cracking have this quaint notion of running a little dictionary file through a program... and this would have been accurate in 1990 for someone cracking at your unix-crypt uni shell account. Today the tools are significantly better and have been refined through the disclosure of hundreds of millions of unencrypted passwords and the same kind of statistical tools that power speech recognition and automatic human language transaction.

- G. Maxwell