This means that first-class marks, when averaged into a mean, can have a dramatic effect on final classification. For a given assessment unit, the difference between getting a bare first and a very good first is the same as the difference between a poor third and a good upper-second. Some examiners are clearly uncertain about how to use the wide top range. So marks in the 70s and 80s are for work that is excellent but not outstanding - a nice distinction. Different kinds of work imply different styles of marking.
Different areas use marks differently.
These inconsistencies affect COGS in particular. The BA programme in AI shares courses with various BSc programmes. However, the issues are by no means confined to COGS, since all examiners need at some time to decide what marks in the 80s and 90s are supposed to mean. Presumably, the committees that determined the canonical marks boundaries had reasons for their choices. However, whilst the decisions have been passed down to examiners and tutors, the rationale has not, as far as I know.