When custom user fields are created, it is possible to categorize them. However, the display of the fields in the signup page is not categorised.
instead, the display of fields club together based on the selection - 'Is this field required' . All the required fields group together at the end of the page - even if they are from different categories.
Ideally, the name of the category should be displayed on the sigup page, and the fields under it should all come together.