The heatmap values are determined by the amount of clicks through to a particular URL. Though it's not based on where the subscriber clicked specifically; hence why the 'heat' is spread out over that content as the data simply reads that those subscribers clicked that content, not the exact position of each click.

The overlay 'Click Heatmap' displays a somewhat useful holistic visual display of clicks but refer to the 'Total Clicks' or '% of Total Clicks' for exact click results.

Switching between overlay views via the overlay dropdown menu: