@Brook *******
here's the back story
before Dec 2018 most people who were using monthly income method simply went to their embassy and got an affidavit of income from abroad attesting they received at least 65K baht a month and immigration accepted that at face value,
BUT
Since the end of 2018 the US, UK and Australian embassies here stopped issuing that letter
SO
immigrations being the benevolent entity they are came up with a rule where those people who couldn't get the income letter any more but wanted to use monthly income could transfer in to the country a minimum of 65K baht a month each month, every month, for the previous 12 months before they applied for their extension to meet the proof of funds.
If you're from a country whose embassy here still issues the income affidavit you get it and use it to show proof of funds but if you're not you do the monthly transfers should you decide to use monthly income method. Many people do it rather than follow the prohibitive seasoning requirements for the 800K baht banked money method.