Perimeter is 1 jump away from Jita, Ashab is 1 jump away for Amarr and Botane is 1 jump away from Dodixie. Each of these systems have one thing in common : they are some kind of secondary trade hubs, just next to their counterpart.
I also noticed that each main trade hub had always a high amount of sell orders, whereas the secondary one was more focusing on buy orders.
Yeah, just to clarify, player owned structures can set much lower taxes. On the flipside, they carry more risk because they can be destroyed, unanchored, or go unfueled. Thus, players are much more likely to set up buy orders with a 1 jump radius there (don’t have put a bunch of assets in the structure, and your buy order can cover both the player and the NPC station), and more likely to keep their sell orders in the NPC station.