The way I think it works is: A skull-level mob is at least 3 levels above you. It could be more. They take the greater of your level difference and '3' and that is what is used. Maybe this is a different thing, but what about 5-man bosses that show up as "world boss" level, as in skulls? For instance, I ran a guild through Uldaman last week, and we killed Ironaya, whose level appears as a skull to my level 70 main.