Quick answer: Turnitin reports run twice. Once upon submission. Once when the dropbox closes.
Full answer: When draft submissions are allowed (before the due date), the comparison of submissions within the same Turnitin Assignment dropbox will not take place until after the assignment due date passes. This means that similarity to classmates’ work will not show in the Similarity Report until the due date.
Reports are run again by Turnitin at the due date, and therefore the percentage similarity may change at this time.
For example, students may notice an increase in similarity after the due date if an assignment template has been used. In a situation where students have used template text such as table headers or report sections, these sections of text will only show as matches within the class group after the due date has passed, and will probably increase similarity percentages for the whole class group. You can exclude sources manually in these situations, when marking.
Important note: Likewise, if two or more students have included the same piece of text, Turnitin will display the existing external database matches prior to the due date; but once the due date passes these student papers will also match against each other.
(This explanation has been adapted from an article from Western Sydney University)