This is less of a coincidence and more of a confluence of events.
Although they've assimilated trillions of individuals, the Borg clearly have a special interest in humanity.
When an interesting set of subjects (the Hansens) present themselves, in the Delta Quadrant no less, they're taken directly to the Queen's Unicomplex for closer inspection.
Later, when the crew of the Voyager try to make a deal with the Queen (regarding Species 8472), the Queen apparently recognises this as an opportunity to smuggle her own operative onto the ship to gain additional information about how best to assimilate Earth and the Federation.
SEVEN: That is why you removed me from Voyager.
QUEEN: That is why we put you there in the first place. You believe that Voyager liberated you from the Collective. Did you really
think we would surrender you so easily?
SEVEN: You've expended significant resources to capture me. Why?
QUEEN: Isn't it obvious? You're going to help us assimilate humanity. We failed in our first attempt to assimilate Earth, and we
won't succeed the next time unless we understand the nature of their
resistance. We want you to be our eyes. Let us see humanity.
Voyager: Dark Frontier
It's not entirely clear how far ahead the Queen planned this. Seven may have been on board any of the three cubes or, more likely, among the hundreds of billions of drones on the three Borg planets.