The Persistent Absence report is set by default to exclude any students meeting an attendance percentage of 90% and above. For this reason, any percentage of attendance that is below 90% - anything up to 89.999% - will be displayed by the report. As soon as a student hits 90% and above, they won't be considered persistently absent according to the threshold set.
If you wish to run a report that does include 90% attendance students, this can done by running the Custom Attendance Report.
When you set the threshold for persistent absence in this report, it will look for and display all students with attendance inclusive of 0% to 90%.