There are a great many science subjects in which marks of above 85 are also extremely rare, I assure you (and have actual data to back up my claims for two of them). The mode of assessment has a pretty big impact on scores, as does the exact material, but the huge range of science subjects means individual anomalies probably get dampened down. I wouldn't be surprised if arts had a very similar Dean's List mark to science. I'd assume biomedicine has a higher average due to the high competition of students for a graduate place and the low number of anomalous subjects like Calculus 1/2 in *first year* that dramatically pull down the first year average.
Idk I'm just going off what I've seen here, in that I know quite a few science first years who scored 90+ in the popular first year subjects (Chem 1&2, the Bios, Physics 1&2, ESD) and extremely few Arts students who scored above 85 in anything (although perhaps Science students just like to brag more

). The quantitate nature of science subjects is one factor, but also for several Arts subjects they actually contrive to give scores below 85-90 (something I've been told by lectures/tutors), for reasons unclear to anyone.