In "Validation Groups" section, just below example configuration, is written:

With this configuration, there are two validation groups:

• Default - contains the constraints not assigned to any other group;
• User - contains the constraints that belongs to group Default (this group is useful for Group Sequence);
• registration - contains the constraints on the email and password fields only.

But has 3 groups in list (Default, User and registration).

I guess a better description would be:

With this configuration, there are two validation groups in addiction of Default