This could be due to the following scenarios:
1) A group has registered with few (or no attendees) and finished the group registration process. The group members that were created in this step will be automatically "complete".
Then, a main group contact has logged back in to the group registration and added a new group member, but did not click "Next" and "Finish" buttons. This would leave the newly added group member as "incomplete".
2) A project administration has updated the state from "complete" to "incomplete".
3) Group members were imported as "incomplete" by a project admin.
I hope this answers your question. If you have any further questions, please submit a helpdesk ticket.