You see email clicks in a lead's activity history but no corresponding web page visits to the Munchkin-tracked page. The clicks happen immediately upon delivery of the email, or sometimes even before the delivery is logged.
This is usually caused by email security software on the receiving email server. The security software tests the links to make sure they are not malicious and this causes Marketo to log a click activity for the email. Because the security software does not actually open the web page in the browser, there is no web page visit logged.
Your emails are more likely to be link tested if your sender reputation is low. For more information on link testing, see the following documents.
Understanding a Spike in Click Activity
Cracking the Inbox Code: Barracuda