I made reservations by Delta to Doha and the flight to Doha was by KLM connecting at Amsterdam. I made my seat selections. You know the rest of the story. The selected seats disappeared as my date of travel closed in. I was annoyed.
What I found was the following:
KLM has a Super Economy seats (God knows what these are as they are exactly the same as others) and they are strewn all over the economy cabin with no logic. If your original seat selection was in one of the forward rows of the economy, you are likely to find that you had been reassigned to one of the back rows.
I think the purpose was to charge more for insignificant improvement and to make people who paid more feel good.