In Section 7.2 "Validation of HOTP Values" of the HOTP spec (RFC 4226) it says, in part,
The HOTP client (hardware or software token) increments its counter and then calculates the next HOTP value HOTP client. If the value received by the authentication server matches the value calculated by the client, then the HOTP value is validated.
I don't understand this. Doesn't the client send the value it calculated to the authentication server? Thus, wouldn't "the value received by the authentication server matches the value calculated by the client" always be true?
What I am misunderstanding here?