when they went to the birthday based system in 1997, people already on SS were kept on the 3rd of the month. I assume because natural attrition will take care of it. Everyone who has started on SS since that time has been on the 2nd/3rd/4th Wednesday system. Except, for some reason, people who also get SSI, and SS recipients in a foreign country.
At a guess, the vast majority of recipients who filed prior to 1997 have now passed on and recipients in a foreign country are about the only ones NOT on the birthday based system.
So, they're cleaning it up.