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It comes from banking. In that context, $6MM stands for the Latin "6 mille mille," or "6 thousand thousands". $6M seems natural for those used to SI units (6 megadollars), but I have heard that in some financial circles that may be taken to mean "6 mille" (what we would call 6k).

It's all probably moot, the average person uses and understands them as interchangeable, I'd imagine.

And to avoid a confusion between a US billion (10^9) and a UK billion (10^12).

Errors of 3 orders of magnitude used to worry bankers once upon a time.

Possibly to avoid the confusion of long and short number scales?

http://en.wikipedia.org/wiki/Long_and_short_scales

M means 1,000. So MM is one thousand 1,000s or 1,000 X 1,000... which is 1,000,000.

While I understand this convention, I personally hate it. Mixing roman numerals and SI units is just dangerous.

(Didn't Mary Meeker vastly over/under-estimate CPMs on YouTube or something like that a few years ago because of this very reason?)