The answer to your "why, why" question could fill a book. It
has to do with character sets and glyph encodings. The short form
is that the curly single and double quotes are not the same
character as the real single and double quotes.
The answer to "how" is much easier. Go into Word > Tools
> AutoCorrect Options. On the AutoFormat tab, uncheck the check
box for Replace "straight quotes" with "smart quotes." Then go to
the AutoFormat as You Type tab and uncheck the same check box
there. Click OK to save this.
Now open your document in Word and go to Edit > Replace.
Copy a single curly quote from the document into Find What and type
a single quote into Replace With. Replace All. Then do it again for
straight quotes. (You may have to do it separately for left and
right curls - I forget.)
Your document should now import without the odd characters.
And spread the word about stamping out the evil of curly quotes in
technical documentation! They are a print standard, and they do us
great harm when we single-source. Wipe them out!