The similarity score of a paper is not a constant set in time. It is based on the similarity at the time of submission based on all submissions in the Turnitin database to date. Once the due date has passed, the similarity report is completely and automatically re-generated for ALL papers in the assignment, thus comparing submissions made by all students in the same class/assignment. The percentage for a student submission may thus change once the due date has passed.
If students are unsure, they can also refresh their submissions. For further information, please refer the students to the eLearning Services guidance on Turnitin Assignments which can be found here: https://roehamptonlearning.com/eLearningServices/?p=4054.