Transformer losses question

This is a "sort of" homework question, but I think it's valid to ask.

I did an experiment with the objective of modelling transformer losses. Naturally one of the tests performed was the open circuit test, so that we can determine core losses (Rc) and magnetizing inductance (Xm).

I plotted Rc and Xm vs. Voltage and obtained a graph I can't explain. As the voltage was increased, Rc decreased. I can't explain this - since Rc models eddy currents and hysteresis losses (right?) I would have expected Rc to increase as V increased (since there would be more current flowing through this element).

The other graph showed Xm increasing with voltage. This makes sense, I think, since as the voltage is increased more current goes through the excitation branch, and hence, we have more leakage flux in the primary inductor.

So I'm rather confused about the Rc results - is a decrease in Rc with voltage, in fact, what we expect or does it look more like I have bad data? (ie, screwed up the experiment :-o)

Thanks for your help