Why do I get a 3-month stamp first when applying for a retirement visa in Thailand?

I think you have your wires crossed a bit

This is how it works

You apply FIRST for a 90 day Non-Immigrant Type O (the O means other) visa based on being over 50 for 2000baht

That goes under review for a period of time (which varies by immigration office from 2 to 3 weeks)

You go back and get the Non-O visa inked in they mark it USED (because you're in thailand) and you get a new 90 day stamp <- because that visa is good for 90 days 🙂

At most offices when you have 30 days or less left on that new 90 day stamp (and your 800K baht is seasoned in the account for 2 months) you go back to the immigration office with all new documentation and apply for a year extension for 1900baht. As a rule extensions are issued on the spot..

That's just how the process works and applying for the 90 day Non-O visa isn't anything to do with applying for the year extension, although they do require similar documentation..

They are two completely different things