I got that question from the staff as well.
I said it might be because of the ratio of Nero d'Avola: you can have a 100% Frappato Cerasuolo di Vittoria, but I believe if you go over 70% or so with the Nero d'Avola, you can no longer apply the doc (both Belfrage and Lynch/Bastianich list the maximum for Nero d'Avola at 60%, but Fia'Nobile makes a Cerasuolo di Vittoria that lists 70% Nero d'Avola, so I don't know). Also, there is a the possibility that one or both, but most likely the Nero d'Avola, is sourced from outside of the zone. Say Pachino, perhaps. And then there is the possibility that some sort of minimum ageing requirement has not been met.
Maybe Kevin McK can chip in with the correct answer.
I will say that many producers in Sicily elect to go under the Sicilia IGT banner for their wines, as they seem to think that DOC designations such as Contea di Sclafani or Delia Nivolelli mean little or nothing to consumers. Marco De Bartoli tried to push through a Sicilia DOC in the mid-90's and his opponents put the fraud squad on him, almost bankrupting him, and perhaps causing the death of his uncle (due to the stress).
But hey, some Prosecco can now be labelled DOCG, so I mean, the system must be working.