The only thing I can think of off hand, and I don't know a lot about it, is that once upon a time, a long time ago, I was told that the redish brown primer contains ferric oxide. I'm not sure if that is true, but if there is enough of it, and you use a thick layer of primer, it might contribute to a hysterisis loss, although I'd be curious if you could even measure it at full power if it did. Rich