EX1, Knee setting and zebra query
 

I have just noticed on my EX1 that whenever I use one of the 2 PPs that have the knee setting on, (to stop the whites blowing out), I get a perfect circle on the middle of the zebras, when the lens is wide open. I know I will never use the camera in such a way, but is this normal. I think I can answer my own question, by the fact that no such thing occurs with any PP with the knee setting off, confirmation would be appreciated.
Thanks.