It is not quite correct to read this data by adding up the numbers because one referring domain may refer to the target with different anchors.
The percentage shown is the number of referring domains that have a link with a given anchor relative to the total number of referring domains.
For example, there are two domains and two anchors. One anchor is presented only on one domain and the second - on both. In percentage it will be : in the 1st case - 50, in the second - 100. The sum - 150%.