Basically, the way rares work on nodes is just a random chance to get one, some certain percentage each time the node is harvested. So the chance of getting a rare actually goes up the more the nodes are harvested.
I find that when I use a harvesting macro and let it harvest everything in the area, I tend to get more rares, than when I restrict the harvest to a specific node.
This is only what I observed, your mileage may vary.