No, it is not your imagination. Differnt brands of charcoal will provide more heat depending on their formulation. There is a lot more "stuff" in some charcoals than others. Kingsford Competition Charcoal burns even hotter then regular Kingsford. And 18 pounds of Royal Oak takes up the same amount of area in a charcoal basket as 12 pounds of Kingsford Comp.
This charcoal basket will hold 18 pounds of Royal Oak briquettes . . .
but only 12 pounds of Kingsford Comp.
But 6 pounds of Kingsford Comp will get me 10 hours at about 240° with 10 pounds of pork butt in my UDS.