A trial exam is a school assessment task
Schools choose and mark trial papers under different conditions. A 78 at one school is not automatically equivalent to a 78 at another. Your trial result may affect your school assessment rank, but the raw trial percentage is not sent straight to UAC for scaling.
What happens before scaling
For most examined courses, NESA combines an HSC exam mark with a moderated school assessment mark. Moderation places each school course group on a comparable scale while retaining the submitted rank order and relative gaps.
UAC then uses the marks supplied through the HSC process to calculate scaled marks. UAC states that scaled marks, not reported HSC marks, form the ATAR aggregate.
What to take from a trial result
Use the trial to identify your current rank, weak topics and exam execution. Do not paste the raw trial percentage into an ATAR calculator and treat the result as a prediction. A school specific rank based range is still uncertain, but it avoids pretending the trial mark is already an HSC mark.
Official sources
NESA: How school assessment marks are moderated
Turn your ranks into a planning range
Compare your school, course ranks and cohort sizes with five historical scenarios. The result is a beta estimate, not an official ATAR calculation.
Forecast my ATAR