Where do you get the 1:1 figure from

Also remember it is a ratio. The US only joined the war in 1917, unlike Britain and France who had been blood letting since 1914. The flu epidemic began in early 1918. Therefore as a % of total US casualties the flu epidemic hit much harder.

The AEF only began serious fighting in late 1917 and it was quite a small force. The US begun the fighting almost as soon as the flu started, whereas GB and FR had suffered over a million casualties already. (Casualty means dead and wounded, in case there is confusion)

If you looked at GB and FR flu statistics as a % of total casualties in the period late 1917-1918 the % would probably be very similar as AEF. However even this would not be fall proof due to British and French offensives, so you'd have to find a period where not much was going on, this is one of the problems with statistics they are often quite misleading and don't really represent the bigger picture.