So, after almost two months (or was it three? I have to go look) I finally got an answer from Kindle as to why my books have DRM on them, even though I have never selected it.
'It happens when you upload in certain formats.'
Note that they did not tell me -which- formats.
So this weekend I'll be reuploading all of my books in an attempt to get DRM removed from them. We will see what happens.