M and — are not always equally wide, And I wonder why, I realize that the EM-dash is one EM, but why is a M not one EM?
You can read about em on wikipedia. I'm basically just summarizing.
Em is a unit which is relative to the font size. One em is the same as the height of the metal sort of the letter. (Even though we don't use metal sorts anymore, we still use the same geometrical principles.)
Traditionally the metal sort for the letter "M" was as wide as it was tall - a square. So that's where the word origins from.
But as the years went by, new graphic styles/schools emerged and it didn't always make sense to make the "M" fit into a square.
Today em simply means a length which is the same as the chosen font size.
So the short answer to why the letter "M" isn't always one em wide is that for many fonts it just wouldn't look good.