As said by others, the correct answer is that worlds within dreams do not need to follow the physical laws of the real world. Dreams also do not need to be logically consistent.
With that being said, a different dream does offer an explanation (Tsunade's), in episode 437: The Sealed Power. In truth, the story is actually a book within a dream, which Tsunade is reading. While talking to Naruto, Minato says he sealed half of the Nine-Tails in Naruto and half in Kushina. Supposedly this was done in order to save Kushina's life, but no further details are given with that regard in this episode.