I think the Base fare is already sold out because when I check my dates in May all I see is Base Plus, Flexible, Full Flexible. The Base Plus is much lower than when I was on the non-Peruvian site. I can't believe the Base price being even lower.
The Base fare is not necessarily sold out for those dates, it may not be showing up because there's a minimum stay or Saturday night requirement or something like that. Try checking out the departure date with a new return date a couple of weeks away and viceversa for the return date and you should be able to see if the Base fare is sold out or not (if it is, it should say "Agotado").