GB, it has always been my assumption, nothing more scientific than that, that most of the scum came from blood in the meat.
If I understand the process correctly, Kosher processing of birds requires the birds to bleed after slaughter and then brines the critter to get rid of the rest (how that does it, I am not at all certain).
Is it possible that your bird was either Kosher or that you brined it?
Maybe that is the difference.
Just a guess.