If the price difference is relatively small, it may just be due to a lack of regulatory fees/taxes. Last summer I bought a ticket that was essentially two roundtrips (spaced out far enough that even if I had bought them separately, they would have been Saturday night stayovers with the same fare class).

It was slightly cheaper, but only because certain taxes/fees are charged per-ticket, and it counted as one ticket. I once tried to get 3 round trips on one itinerary, but Silver line couldn't do it (they tried, but the computer kept rejecting it).

But if your price difference is significant, than this is a great thing to keep in mind of folks who go back and forth with plenty of notice between two places.