Aisha Greyjoy

I have 7 cities at 1200 infra, using nuclear power.
 
At 1.2 per 1k, I could be using 8.4*1.2 = 10.08
 
I actually use 16.8.
 
An alternate explanation, offered above, is that 7 1k cities would use 8.4, and 7 1001 cities would use 16.8.
 
That exactly fits my situation.  We assumed(I did too) that nuclear power scaled downward, that 1100 is 10% more uranium use then 1000.  In fact, the first 1000 uses 1.2, and anything between 1000.01 and 2000 uses 2.4. 
 
Grillick's explanation is correct.