Read the original ASCII specification. It has nothing to do with fonts or how people drew glyphs as Mr mgk25 seems to imply. Granted many developers drew them this way, but it was because it was more aesthetically-pleasing.
ASCII only had seven bits to work with so some characters carried multiple meanings. I never see people complain when - is used as both a hyphen and a minus, or when people use it in place of the Unicode hyphen/minus/dash characters.
` was defined as both LEFT SINGLE QUOTATION MARK and GRAVE ACCENT. `This' and ``This'' are the proper quoting methods in ASCII. They pre-date TEX and /prog/ and wherever else you may have seen them crop up. Typing 'this' is equivalent to using parentheses )like this) instead of (like this). If you're so obsessed with symmetry, use the Unicode quote characters (‘ and ’, “ and ”). Don't do 'this' then preach me to me when I use characters correctly.
CONTEMPORARY QUOTATION MARK USAGE CONSIDERED HARMFUL