Word automatically changes some hyphens into em- and en-dashes (an em-dash, as the name implies, is the width of a lowercase m, and an en-dash is the width of a lowercase n). For example:
Type a letter, followed by two hyphens, followed by another letter, and Word changes the hyphens to an em-dash. It's nicely done, because the em-dash has a little bit of space to the left and right, and a line can break before or after an em-dash.
Type any letter, followed by a space, a hyphen or two, and any other letter, and Word transforms the hyphen(s) into an en-dash.