CPSLO does not consider “level of applicant’s interest” according to http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=666 , so that reason to use ED is not applicable there.
The description of CPSLO’s admission process and criteria in http://admissions.calpoly.edu/applicants/freshman/criteria.html and http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1014&context=statsp does not show any place where applying ED could give a benefit in terms of chance of admission.
It does seem possible that if they underestimate the threshold for admission to the school, division, or major in the ED round, they could admit some ED applicants who were below the threshold that is finally determined after seeing all of the RD applications.