That can happen in a number of ways. The most common one is that one of the pair that's headed inside is intercepted by another villager on the way to the hut and goes off to do something else. Another common one is that an elderly villager mates with one much younger, and takes f-o-r-e-v-e-r to get to the hut. Anything could happen along the way when you're moving like a snail!