Very nice Logic!
I cannot say for sure, but, if Steve had an accurate meter reading true RMS, then it is possible that the extra wattage discrepancy despite lower duty cycle could be from electron collisions.
I doubt I will ever see the MEST or CEST software (as they are NASA tools, and carry a hefty price tag)....but I did find this.
Going over the document now, but it looks like it could help out.