I know that the formula for the entropy of a message is: entropy formula (I believe b can be set to 2)

but when calculating the entropy of a password websites usually only compute log2(p-1), ignoring the first term. Is there a difference in these types of entropy?

Note: The website I am referring to in this instance is http://passwordstrengthcalculator.org/index.php. Thank you for the help.

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