Iberia is weird

So i go to Iberia.es to book some flights. I try to get Barcelona->Paris (just one way) for 30th November. Price is 632 €. OUCH. Then i try Barcelona<->Paris (roundtrip) coming back home the 2th of December and it costs 136€. Errr, what?

Anyone with insight in how flight companies work can explain that nonsense? Or is it just that French hate us and they let us go to do some tourism and come back but not just go? :-D

Update: The french hating us is a JOKE. I've been twice to Paris and it has been great.