what do you mean by so many opportunities to lose marks when you try to move up from D to HD?
To illustate what he means I'll use an example. Say a subject has the following assessments and you get the following marks.
Assignment 1 (Weighting 30%) Mark: 80%
Assignment 2 (Weighting 10%) Mark: 90%
Assignment 3 (Weighting (20 %) Mark: 88%
Exam (Weighting 40%) Mark: 83%
So your end of subject score is 84, and you get a D* (24+9+17.6+33.2=83.8 which rounds up). While you have got good marks always in the D-HD range, (in this case your average mark is 85.25) you only achieve a D because you performed worse in the assessments with higher weightings (the more important assessments).
Also to get a mark of 85% in an exam/assessment in uni for most people requires a far greater amount of work than what effort would be required for the same mark in high school. Plus assessments are usually designed so that only a small number of people get HDs etc.
*F=0-44
PC=45-49
P=50-64
C=65-74
D=75-84
HD=85+