Assuming you haven't accidentally chosen an Iberia service, I would guess that it's because of the mixed-class itinerary.

You're given different Avios > Cash ratios and quantities for Business vs Economy, so when it's a combined itinerary, it probably isn't capable of working out a 'rate' for you.

See this link for the usual payments amounts for the two different classes:
https://www.britishairways.com/en-gb...s/part-payment