She's simple. But Shinichi is not. So she believes him since he is a case freak. There are no other possible explanations which can satisfy Ran. So she believes.
Will SHinichi ever tell Ran about his Conan identity?
I am thinking of a thought that is actually not a thought but is a thinking about things that has some thing to do with the thought of being random and that is the Random Thought we are thinking about right now...