I am not talking about frequency changes varying Ra. Ra varies, from the characteristics chart, vs current at any frequency. This varies more over the half cycle from idle current on down VS from idle current on up, where Ra tends to be much more constant. So what you might be measuring is the Average DF or output Z. But since Ra changes over a portion of the cycle, the output Z also changes over that portion of the cycle.

When the higher output Z occurs, there is less control of the woofer/midrange/tweeter, not only to dampen, but also for initiation of a signal.

Take care.
Steve