Others here have covered the fact that M means thousands to many people. But where does it come from?
As best I can tell, it comes from the fixed-income markets, where MM/MMM/MMMM have been used for many decades. This convention is not universal in finance--you often see monetary policy news using M/B/T instead. And applications not targeted specifically at finance professionals often use M/B/T (e.g. Google/Yahoo Finance). The most widely-used finance application after Excel is probably the Bloomberg terminal, which uses MM/MMM/MMMM for all asset classes (n.b. Bloomberg originally was most focused on bonds, so of course they adopted that convention).
Once you have arrived in the present situation, where M can mean either thousands (to the weird bond people) or millions (to laypeople), you realize that MM is understood as millions everywhere, if not right away. That's better than ambiguity. And B for billions is a little wonky anyway, because not every language calls it a billion (the Dutch, French, and Polish all start their "billions" with an M).
As best I can tell, it comes from the fixed-income markets, where MM/MMM/MMMM have been used for many decades. This convention is not universal in finance--you often see monetary policy news using M/B/T instead. And applications not targeted specifically at finance professionals often use M/B/T (e.g. Google/Yahoo Finance). The most widely-used finance application after Excel is probably the Bloomberg terminal, which uses MM/MMM/MMMM for all asset classes (n.b. Bloomberg originally was most focused on bonds, so of course they adopted that convention).
Once you have arrived in the present situation, where M can mean either thousands (to the weird bond people) or millions (to laypeople), you realize that MM is understood as millions everywhere, if not right away. That's better than ambiguity. And B for billions is a little wonky anyway, because not every language calls it a billion (the Dutch, French, and Polish all start their "billions" with an M).