More like, students who graduate one semester early or one semester late are likely to have an extra fall semester. Of course, it is likely that about half of drop outs have an extra fall semester. The desire to balance fall and spring enrollment (particularly those enrolled to capacity, like many state universities) is the incentive for colleges to have spring start admission.
Study abroad and off-campus co-ops should have a neutral effect on fall versus spring enrollment, if equal numbers of students do them in the fall and spring.
If the college allows spring start students to take transferable community college courses in the fall before the spring start, that can be a way for the student to reduce some of the costs (i.e. 1 semester of CC + 7 semesters of four year school may cost less than 8 semesters of four year school).