It suggests that you are confusing the type descriptions with the actual type. Reasonable people may differ on what constitutes type, but in my opinion, MBTI type is better defined in terms of "how you think", and not "what you appear to be". Visible personality changes over the years, especially in childhood. INTJs act just as silly and childish as ISFPs, at that age. And when the INTJ matures and the ISFP matures into their 40s and later, neither one is especially childish at that point. But the paths that each of those types take from childhood to adulthood strongly differs.
INTJs start out very childish and silly, but as they learn - and especially once they get to the parts of their education that emphasizes NT-style reasoning over SJ-style reasoning - they start to shine and excel in ways that the ISFPs tend not to. Not that that ISFPs do poorly, necessarily, but academics is generally not the focus of their type.
Interestingly, something odd happens as these two types age into later years. The INTJs tend to gradually start pursuing very ISFP areas of interest, while ISFPs start looking at education and technical know-how much more closely. Personally, I started getting interested in dancing, and have been an avid dancer for several years now. An ISFP of my acquaintance was (and is), a wannabe rock star, regularly plays in local bands and so on, but when he realized that wasn't a money-making pursuit, changed tack and began to excel in being able to explain complicated software (e.g., PeopleSoft) to his employer's clients. Now, I didn't become an ISFP: my approach to dancing is far more analytical than that type would ever tolerate (I dance in my head!), and my acquaintance didn't become an INTJ. But we both grew into adopting more of the traits that are associated with one's opposite type.
Opposite how? INTJ functions are Ni-Te-Fi-Se, while ISFP functions are Fi-Se-Ni-Te. Given that MBTI is really just classifying the first two functions, that means an INTJ has an "inner ISFP", and the ISFP has an "inner INTJ". These opposites are kind of our "shadow selves" that we create as we emphasize our type. INTJs work on eliminating their ISFP-ish traits even as they develop their INTJ-ish ones, and vice versa, but the shadow is always there, and it's there because it is suppressed. (This is one of Jung's key insights, in my opinion.) As one matures and gradually learns that one's primary approach isn't suitable to all aspects of life, one gradually adopts the methods that one previously shunned, and becomes a much better person thereby.