This question already has an answer here:
I expect this has been asked and answered elsewhere, but I couldn't find an exact duplicate.
The multi-leg return fare:
CWL > AMS > PDX > AMS > CWL
is significantly cheaper than:
AMS > PDX > AMS
KLM's booking office couldn't explain to me exactly why, other than to say it's how the fare structure works.
I assume the airlines have good economic reasons for this, though I can't imagine what they are.
And I also assume that the fare system and its terms and conditions have been carefully engineered, for whatever reason, to make a cheaper fare for the single leg unavailable.