Why are the acceptance rates for publications in subfields higher than others? For example in computer science, systems conferences have low acceptance rates hovering around ~20% as listed here: https://www.cs.ucsb.edu/~almeroth/conf/stats/

But in robotics many of the conferences, even the major ones, seem to have acceptance rates of ~40% (ICRA, IROS) as seen here: http://www.adaptivebox.net/CILib/CICON_stat.html#ICRA

Why is this, and are any steps taken to equalise the playing field between subfields? Does this simply mean that those doing systems research are just going to have a harder time publishing than those in robotics?
You can't judge a conference by its acceptance rate because you don't know what is in the rejection pile. For example, I know of a computer science conference that regulates its acceptance ratio by determining it in advance and then just setting the number of parallel tracks to fit.