Hey Deanzor, I have question for you about this.
Just from my interpretation of this description, it sounds like enemies on the 3DS are prevented from spawning in town by backwall/fence placement, and that having a gap in these defenses would allow enemies in. On PC, enemies are automatically prevented from spawning (even outside, with no backwall) by the presence of enough NPCs (with the exception of invasions).
If the mechanics are entirely different between the two versions, and require people to build additional walls for protection, it could be leading to some bug reports by people who are traditionally PC users. Or perhaps I'm just reading into it too much. ^_^ Some clarification could be insightful, however!