Let’s assume that Kage attack stat is A and opponent defense stat is D. Now let us assume that he does X damage at 450%, and Y damage at 225%.
Therefore, according to damage formula known to us,
X = 100 x (4.5 A / D) ^1.35
Y = 100 x (2.25 A / D) ^1.35
Hence, X/Y = (4.5/2.25)^1.35 = 2.55 (approx)
In your case X/Y = 746/312 = 2.4
The difference comes from the RNG parameter in the damage formula. Hope this helps. 
For more details on this formula check this amazing post.