Don’t hold me to the exact numbers, but this story is reflective of what you say.
According to the Department of Defense, there were 9,000,000 Vietnam era veterans. By the time the 1990 Census was released, that number had swelled to 11,000,000!
People were petitioning the FEDs to have names added to the Vietnam Memorial. The US kept revising the start date backwards, so some of those names were added...
In 1998 the Feds decided that the official start date was November 1, 1955.
(I don’t know if it’s still considered the official start date.)
So I can understand how the number of Viet Era vets increased.
Some say the war started in 1954. The first casualty was considered to be in 1945 - Lt. Col A Peter Dewey.
For a Viet Vet to be eligible for VA health benefits, the VA considers In-Country service between January 9, 1962, and May 7, 1975.
The VA now considers service in Korea during the Viet-Nam era as being eligible for Agent Orange disability benefits.
So when did the war start?