I read that some colleges do a poor job of tracking their graduates, and therefore show a lower graduation rate than is really the case. Many students will graduate from a different college, which may not show up in these stats, if they are not properly tracked.
The 4 year graduation rate is particularly an issue if a student is dependent upon need-based or merit-based aid that is limited to 8 semesters. Even one extra semester without any aid can hurt financially.
The colleges with the highest 4 year graduation rates often have students who are arriving their freshman year with a large amount of AP or dual enrollment credit.
In addition to programs such as engineering that may require more than 120 credits, nursing may also require additional credits. My daughter’s nursing program requires 128 credits, and many students need 4.5 years to graduate. Some start in the 4.5 year program, and others fall into it if they fail a class.
There have been stories in the past about some California public universities where students could not get into classes that they needed to graduate on-time. Many colleges are able to avoid this problem by having enough sections and because seniors are given preference in registration.