The very short answer is that we don't know. She wasn't the only Wolf 359 survivor to end up in the Delta Quadrant. There were at least two others that we know of, and presumably many more given the various Vulcans, Bajorans and Klingons seen on Borg ships.
We know that Borg Cubes sometimes contain secondary ships like the sphere seen in First Contact and we know from Voyager that even small borg ships can access transwarp. The most likely explanation is that the Cube, as well as heading purposefully toward Earth was also picking up samples (via assimilation), then loading them onto scout ships to be packaged up and sent back to the Delta Quadrant for further study(?) and to be used as forced labour on their ships.
As you can see from this still from VOY: Collective, the Borg don't just assimilate people, they also assimilate their ships and presumably re-purpose them as slave transports. This would go some way to explaining how their home planet has 50 trillion inhabitants.