Psychology is a pretty broad term in reality, so I think they differentiate them for people who want to pursue the research and for people who want to pursue practice. They can and often do intersect, but some people go into the field strictly to pursue one or another.
I don't like doing math or science, but I can read and understand case studies and research papers (including the statistics used in them). So, I'm in the Arts program because I'm more interested in how that research affects things in the field.