Why do my buttons show double clicks in the clickthrough heatmap report?

When I view the Clickthrough Heatmap reports, some of my emails show two tags on buttons.  What causes that?

The reason that some buttons show two tags is due to Outlook recipient clicks. Outlook has different rendering rules than other email clients. Because of this, we wrap button links twice. One link is placed on the text of the button and the other link is placed on the surrounding "button" area. For the same email, people who are using Outlook have a slightly different clickable area than other email clients.