You are comparing estimates "300,000" with real numbers "400,000". The "400,000" number comes from records of prescriptions issued to both Union and Confederate soldiers during/after the Civil War.
The 400,000 are real numbers. The "estimate" is not.
Drawing conclusions by comparing real numbers to estimates (really? A conclusion based on two data points? What are you five or something?) is nonsense.
But in another drug thread DiogenesLamp said, "Reliable or not, those are the figures being put out by the people who make it their job to come up with such figures. No doubt they are a statistical extrapolation. Till someone presents an argument that there are better figures, I will have no choice but to use what is available."
These guys were civilians in 1880 and had dispersed around the country. How did they get such an accurate civilian count in 1880, but not in 1900?