I don't get why they wouldn;t give the rounded total score
Uh, so that it can differentiate between applicants and rank them in order of 1 to ~700.
This is useful in the following, rather common scenario:
One person declines an offer, so the next person on the merit list is offered a place. However, there are not one, but three people with the same score for UMAT and who scored the same overall in ATAR and/or interview. Now the uni has a choice here - either it can just not offer that place to any of the three people, or it can wait for two more people to decline and then offer those three people a spot, or it can over-offer by two places.
However, given some new information from ACER in regards to their scores and how one person actually got slightly higher than the other two by less than a whole mark in total, the uni is then able to just offer that person the place.