A = Americans lost on Western Front
B = British lost on Western Front
C = Germans lost on Western Front
You need to compare A+B to C.
I can only presume you're misreading the graph by comparing just A to C, and just B to C (or worse lumping in the Germans lost in early campaigns as well).
Let try to clear up your confusion on the graph:
Does that make is a little clearer?
Adding in the other German opponents really only pads their stats and increases the ratio. That they did it with the limitations you noted makes it all the more remarkable.