First time poster here

I'm looking into booking international flights and I'm wondering why it's so common to see single ticket prices as being more expensive than return/round-trip ticket prices? I'm using the same outgoing/departure date in both queries.

In some cases, the single ticket price is double that of a return. What's the reasoning behind this? And more importantly, can the airline hit me with a no-show penalty fee if I didn't use the second part of my return ticket?