I don't exactly know why. Perhaps the anime writers are less in touch with human emotions vs social defenses, or they just simply decided it would make for better ratings? I cant say. What I can say is this- wait let me get my soap box, I keep it around here somewhere for these very moments. There we are... Okay, now where was I? While the native Japanese, and later English dubed versions show her as a dim witted fool, the manga itself shows a very different and complected individual. In the manga series itself Ran shows she has a kind heart, and a very playful demeanor, yet she has moments where you get glimpses into mind. She is very resourceful, and fiercely loyal in a way one could almost compare to a momma bear. Readers get to see her own inner conflicts, and struggles and it very quickly become apparent that her overly bubbly idiot side is more or less a front for the better of those around her. Watch the anime, and she is someone you couldn't ask to babysit water without her somehow burning down the house.
Ranny is far from the only character in this- or other series to get this treatment. It's painful to watch sometimes and see complete personality shifts between scenes. Those moments where you know instantly “ this is from the manga- and this part is purely TV added fluff” Perhaps it boils down to just trying to make things more appealing to a wider market- or perhaps the people in charge of writing, and adapting the series for TV need to spend more time around real people; hard to say. That being said I want to repeat myself one more time. Ran is NOT the idiot TV makes her out to be! There I said it, now I can get off this soap box and go do something more produ- * trip-CRASH *