You can perceive them because they 'burn into' your retina and leave an after-image. Your eyes act as a queue or buffer of sorts; by the time your brain registers the character (especially characters outside the latin alphabet, and arabic numeral system- characters your brain sees and recalls much less often), several other characters have already flashed by.

You don't notice this effect with contiguous video, but 25fps becomes more of a problem with high-speed action flicks and their ilk.