Person A was sent an email from Marketo, clicked on a tracked link in an email to a tracked web page, but the web page activity was recorded in the log for Person B.
This can happen if Person A and Person B are sharing a computer. Prior to Person A clicking on the track link in the email to redirect to the tracked web page, Person B was already 'cookied' as a known lead on the same computer and web browser. In Marketo, once a lead becomes known, cookied and tracked on a web browser, any tracked web activities on that browser will be associated to the first cookied lead until the cookie is expired or deleted.
Note: to confirm that Person A and Person B are using the same computer and web browser, you can check the details of the most recent web activities and compare if their User Agent and Client IP address is the same.