Topic on Forum:Trope Talk
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It's hard to say, but I'd wager it's just the cumulative effect of thousands of different people editing the articles over the years. Last time I looked, several years ago, TVT had 60,000 registered users. If only 1% were actively editing the wiki, that's still 600 different voices and writing styles. It's not surprising that the average is going to sound a little odd.
And naturally, since we forked off of TVT, we inherited all those oddities.