From past year reports, UC applicants get notified by UC after putting in their courses and grades into the UC application (which then recalculates GPA and compares it to the threshold GPA for their high school).
Since the determination is based on a known method, the high school can do this as well and inform students of their ELC status. However, if the student has taken courses outside of the high school but not reported to the high school, then the high school’s determination of ELC status may not match the actual UC determination of ELC status that includes all high school and college courses completed.
This is likely for the purpose of calculating the threshold GPA for the high school.