In a mileage run thread recently, someone listed a JFK-ONT route for around 4 cents per mile. I looked up specifically JFK-ONT on different wknds, and then attempted to add nearby airports to see if any more convenient airports would yield better pricing. For most weekends, the results shows no overlap and I'm wondering what I did wrong.

For example, Sept 21-23 - JFK-ONT. Clicking only 2-3 stops, a bunch of $400ish routes come up. When I add nearby airports (still including JFK and ONT), those aforementioned $400ish flights do not appear. What's going on here? I am afraid that if certain routes are missing on this specific search, this is happening in general as well.