A nice friendly agent at AMS told me what she believes is the real problem here. She said I was given my C+ seat when I booked but when check in opens for the local AMS folks if they want to BUY a C+ seat, KLM will sell them one and bump me out. I was shocked to hear this and didn't believe it until I thought back to the 3-4 times this has happened to me and it makes total sense.