The character () is used for both a typographic apostrophe and a single right (closing) quotation mark. The fact that they share a single (Unicode) codepoint is because many computer fonts and character sets (such as CP1252) had previously unified the characters into a single codepoint, and the difficulty of software (at the time) being able to distinguish which character is intended by a user's typing.
The straight apostrophe (the "ASCII apostrophe", ) is even more ambiguous, as it could also be intended as a left or right quotation mark, or a prime symbol.
See also Modifier letter apostrophe General Punctuation (Unicode block)
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