I think it is because of more thorough wiping done by the US D press. It is harder to see in person, but the scan above shows a bluish tinge to the background of the French stamps. I brought up/the blue in the US stamp a little and found there is even a bluish tinge to the background there. The result overlaid over the two blocks:
I think the blue matches the French issue well, as does the red. The slightly more bluish background might be responsible for a different perception of the inked colors.