I know. It still doesn't make sense when people use k/M for 1,000, M/m/kk/MM/mm for 1,000,000 and B/MM/mm for 1,000,000,000. (sorted by ~popularity)
It's even more annoying when someone uses "k" and "mm" in the same post, since the whole idea of "mm" comes from repeating roman "M" (1,000). So why not stick to k/kk (WoW-style) or M/MM (almost-roman style)?
At least "k" and "M" are based on some kind of standard.
From the link: "It is commonly abbreviated as m or M; further MM [...], mm, or mn in financial contexts." Note the semicolon.
Also note that the only cited source additionally claims that "M" is often used to indicate one thousand, as in $60,000 = $60M, which for all I know may be true in the financial world but is definitely not true for anyone outside that world who wants to be understood.
As long as we're being pedantic, abbreviation does not refer strictly to "subtractions of letters existing in the word," it is any shortened form of a word or phrase.
It's even more annoying when someone uses "k" and "mm" in the same post, since the whole idea of "mm" comes from repeating roman "M" (1,000). So why not stick to k/kk (WoW-style) or M/MM (almost-roman style)?
At least "k" and "M" are based on some kind of standard.