Thanks Salvo. The IQ4 blurb does state that it uses battery voltage, not converter voltage. It says Bulk voltage is 14.8 and the high trigger is 14.6
"... This BULK STAGE will continue for either 225 minutes or until the battery voltage reaches the “High Trigger” value (whichever occurs first). At this point, the BULK STAGE will operate for another 15 minutes before switching to the ABSORPTION STAGE."
On the amount of spread I see, using the voltage drop calculator, I do get 0.2ish drop at 100a as being likely, so that solves the mystery of why I get so much and the PD9280 only needs a tiny spread.
So we have the actual voltage difference spread to make the amps and also a voltage drop to make a total apparent spread like I see.