This means that the top 13 unique mask structures make up 50% of the passwords from the sample. Over 20 million passwords in the sample have a structure within the top 13 masks.
Based on analyzing the data, there are logical factors that help explain how this is possible. When users are asked to provide a password that contains an uppercase letter, over 90% of the time it is put as the first character. When asked to use a digit, most users will put two digits at the end of their password (graduation year perhaps)