Grappling in a gi - or even without - is not the same as MMA. The entire dynamic changes once strikes are included. How many times have we seen a BJJ world champion not only lose but get submitted? Clay Guida did it the other week and Rodolfo got submitted prior to that.
In a straight grappling match, there would be less disparity between Dustin and Charles’ BJJ. The problem is, the setups required for sweeps and submissions in BJJ don’t work so well when someone is raining down punches on you. A black belt can quickly drop a belt or two down in grappling ability when you factor in strikes.
It also may explain why the Dagestanis who train in Combat Sambo - grappling + strikes - are usually more successful in adapting their grappling to MMA than pure BJJ guys.