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.

via Statistics Will Crack Your Password.

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)