Good point; it's been so many years since I last watched that story arc that I'd forgotten.The Borg smash 39 starships at Wolf 359 and capture many crewmembers. The Borg Borgify the crewmembers and sail to Earth with the Borgified crewmembers aboard their cube. The Borg cube explodes near Earth (hopefully not too near or there might be an Endor holocaust on Earth), and all the Borg and all the Borgified crewmembers are vaporized. Later Voyager in the Delta Quadrant tens of thousands of light years from Earth encounters some of the Borgified crewmembers from Wolf 359 that should have been vaporized with the cube at Earth. That is the problem. Why are the Borgified crewmembers still alive and still Borg in the Delta Quadrant instead of being clouds of incandescent vapor in space near Earth?
I guess the best in-universe explanation is that some of the newly-assimilated were transferred off-screen to other Borg vessels that weren't vaporized, and returned to the Delta Quadrant.
Of course the more likely explanation is that the writers didn't care about checking for continuity, which is why they forgot about the Borg baby by the end of the episode.
Or... it could just be another alternate timeline.