Can someone explain why MM means millions, and not just M, 3M ==3 millions?

https://accountinginsights.org/how-to-abbreviate-million-in-...

  The abbreviation “M” for million can lead to confusion in finance. Historically, “M” derives from the Latin word “mille,” meaning “thousand.” As such, it has traditionally been used in some accounting or construction contexts to denote a thousand. For example, $5M could historically represent $5,000, creating ambiguity.

  To represent one million in finance, the abbreviation “MM” is widely used. This notation originates from “mille mille,” meaning “thousand thousands” in Latin, equating to one million. This clarity makes “MM” a preferred choice in financial statements and reports. 

  Other abbreviations for million, such as “mn” or “mln,” are also encountered.. The Financial Times, for example, adopted “mn” for millions to improve accessibility for text-to-speech software. While these alternatives exist, “MM” remains a prevalent and widely understood abbreviation for million in American finance.

Because there are too many standards[0]! If only someone would invent one more standard that everyone could follow...

The Wikipedia on long and short scales[1][2] for the root of the problem.

[0] https://xkcd.com/927/

[1] https://en.wikipedia.org/wiki/Long_and_short_scales

[2] https://grammarhow.com/m-vs-mm-million/