According to the article the breakdown of the 7.5M is 2.2M for the Air Force, 3.3M ground forces, 2.0M supply and training. That last part doesn't make sense to me. I thought it took more than one rear echelon type for every actual combatant.
This may have been hype for the press.
"The Secretary of War said he was making public the figures on the Army because of the circulation of estimates ranging from 10,000,000 to 13,000,000 which did not come from the War Department. His was a conservative estimate, he added, 'and any changes will be upward.'"
It may have depended on what is is. Lots of numbers thrown around. Some numbers may have included civilian numbers. Military would catagorize differently. I assumed divisions, but there was lots of support. Divisions are more common to army and marines. Airforce was a part of army then. I don’t think they completely knew, it was a moving target.
But it was a heck of a lot of troops and and tremendous organization effort.