It is possible for students to still submit answers after the assignment has closed.
Example: If they started the attempt before the closing date but then took a long time before submitting.
These responses are stored, but the students are not given any credit for them. The instructor can see these answers when reviewing the student's attempts and can give the student credit for them manually. Also, they could change the closing date after the fact and regrade the attempts, making the student's answers graded.