UGC publishes Z-score values after GCE A/L examinations based on the district you sat the examination on that particular year. But your district applicable to university selection is based on government policy on the admission of students to Universities.
UGC (University Grants Commission) select students to National Universities based on your period of study (Highest number of days during last three years) in the district where your School is located just prior to A/L examination.
FAQ section of UGC website (www.ugc.ac.lk) describes the method of determining the district of a candidate.
What is the method of determine the applicant’s district for university admission?