Here's my still-in-the-research-and-learning phase opinion. I think adding the charcoal allowed you to get the blade more consistently heated throughout, possibly a little hotter than you were previously getting because of the open chamber of the block, and a tad hotter when it hits the quench. The charcoal takes up space, preventing as much airflow within the chamber, while at the same time retaining more quantity of heat allowing the steel to be in the austentising (sp?) temp range longer, having more carbon in solution when it gets quenched and transforms. You know what I mean....
I personally don't go for the absorption of carbon idea. While carbon is present in so many things, the carbon present in knife steel composition is different from the carbon in used motor oil that is used as a quenchant, different from the carbon in charcoal, etc. If you could actually add (or change) the amount of carbon in the steel, you'd never be able to get it heat treated because you'd never know how much was "actually there" and it would be a new hybrid every time.
Granted this is all just my opinion from the research I've been doing (trying to learn as much as I can) but what I think occurred is the dynamics of your HT procedure changed enough to alter the transference process, albeit in a good way.
Regardless, I think it's awesome! It will be cool to see if you can duplicate it again next time, which I'm sure you're already thinking about!! Since the target as-quenched Rc is 64 for O-1, you couldn't be any closer and your HT is better...all very cool!!!!!!!