An Unsubscribe activity is logged when the lead clicks the system Unsubscribe link embedded in a Marketo email. However, if that link takes them to a Preference Center that allows them to select from multiple options, they may not choose the full unsubscribe option and may continue to receive selected emails.
To confirm if this is the case, find the Unsubscribe activity in their activity log and double-click on the activity to open it. This will show you the choices they selected in the email preferences form. In the example below, the lead did not fully unsubscribe, they only chose to unsubscribe from specific content. Since they did not choose the global unsubscribe, they will continue to receive some emails.