How is this possible?

Almost all new airports are old airports that were not official.

Insurance is more nearly affordable if your airplane is limited to
operations at FAA designated airports. Some owners have been finding
it cheaper to make their airports official than pay for off-airport
landing insurance.

Also, lots of local governments have been putting pressure on private
airport owners to bring the facility "up to code" and make it an
official airport. Most keep them private, but some actually go public.

I'm personally aware of 3 instances of this in Michigan.

So, I'm thinking the AOPA doesn't count them because they're not new
airports, they're old airports with new paperwork. But that's a guess.