I'm not 100% how UNSW grading works, is it distributed so the rank would indicate a higher grade (e.g a HD/D) or would a 51 just = a pass grade with this rank not being taken into consideration.
For most courses, assessments/assignments are simply raw percentages, so knowing the rank for a small task is kind of redundant. There might be some courses where individual assessment/assignment are scaled, but usually it's the final mark that gets adjusted.
There is no requirement for a higher rank to be scaled to a certain grade. There's no bell curve distribution, but individual course conveners may decide to use such a grading system. There is a committee who decides whether the distribution of marks is appropriate and may decide to raise/lower/keep the same marks depending on any number of factors (that are unknown to us).
Having been in courses on both sides of the marking spectrum, i.e. courses with lots of raw HD marks, and courses with lots of raw PS marks, the OP achieving 51% and being in the top 8% of the cohort for 1 single assessment, isn't very surprising.