There are many contributing factors, but I expect the most influential one is a higher rate of applicants. Students attending selective private high schools in the northeast are tremendously more likely to apply to selective private colleges in the northeast than the general population. If the well qualified kids from a particular HS do not apply to Ivies, then that HS will not have many students attending Ivies.
The 2nd most influential factor is probably having a higher rate of well qualified students among the HS population. This often relates to the HS being highly selective. For example, the highest ranked HS on the IvyLeaguFeeder website is Trinity, which is a selective private HS in NYS. To get into Trinity, students need to do well in an admission system that involves grades, test scores, LORs, essays, hook status, etc… a system that has a lot of similarities to the Ivy League admission criteria. Students who are accepted to Trinity are pre-screened as kids who do extremely well in an Ivy-like admission system. According to the Trinity school profile, the middle 50% ACT among Trinity students was 34-35 – higher than Harvard or any other Ivy. The lower end of the class at Trinity is the 99th percentile among HS students overall.
It is likely that the vast majority of the class at Trinity is well qualified for Ivies, which is very different from typical non-selective HSs. I attended a non-selective public HS in NYS, where hardly anyone has test scores as high as Trinity kids. If you compare students with similar stats and hook status from Trinity to my HS, I’d expect the students from the basic public HS I attended who apply to Cornell have a much higher acceptance rate than does Trinity. My HS doesn’t have a large enough sample of kids who apply to Ivies other than Cornell to speculate on other Ivies.
As has been noted, another relevant factor is different rates of hooked kids. The top ranked HSs tend to have higher rates of ALDC type hooks than the general population, although the overall HS population generally has higher rates of URM hooks than highly selective private HS.
In my opinion being a so called “feeder high school” is generally far less influential than any of the above. In many cases, attending a “feeder HS” offers no advantage in chance of Ivy admission over non-feeder HSs. For some students, attending the “feeder HS” can be a disadvantage, such as kids do better as big fish in a small pond.