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I comes down to a basic and simple deal: most people read quickly and allow our brains to fill in the short words (we don't actually read them each time). Therefore, a cursory scan of the page shows 3 letter Fs, because there are only 3 in words with more than 3 letters in them.
It isn't an IQ test, it is actually more a test of the speed of your reading (if you saw 6, you probably read slowly!).
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